How can a glider pilot add sport pilot privileges?
The short answer is no. You can get student pilot privileges with only a CFI endorsement for a new Category and Class. You would not have Sport Pilot privileges, though. The regs for Sport Pilot certificate holders would be that you need endorsements from two different CFIs and file an application with the FAA for each Category and Class. 

Although 61.303 and 61.321 can be confusing, they both make mention of category and class. A sport cert does not have Cat &amp; Class. For Sport Pilot privileges you have to have a training endorsement from one instructor and a proficiency check endorsement from a second instructor before the FAA will issue you a Cat &amp; Class endorsement. 


  61.321 How do I obtain privileges to operate an additional category or class of light-sport aircraft?
  
  If you hold a sport pilot certificate and seek to operate an additional category or class of light-sport aircraft, you must—
  
  (a) Receive a logbook endorsement from the authorized instructor who trained you on the applicable aeronautical knowledge areas specified in §61.309 and areas of operation specified in §61.311. The endorsement certifies you have met the aeronautical knowledge and flight proficiency requirements for the additional light-sport aircraft privilege you seek;
  
  (b) Successfully complete a proficiency check from an authorized instructor other than the instructor who trained you on the aeronautical knowledge areas and areas of operation specified in §§61.309 and 61.311 for the additional light-sport aircraft privilege you seek;
  
  (c) Complete an application for those privileges on a form and in a manner acceptable to the FAA and present this application to the authorized instructor who conducted the proficiency check specified in paragraph (b) of this section; and
  
  (d) Receive a logbook endorsement from the instructor who conducted the proficiency check specified in paragraph (b) of this section certifying you are proficient in the applicable areas of operation and aeronautical knowledge areas, and that you are authorized for the additional category and class light-sport aircraft privilege.


Private, Sport, Recreational, Commercial, etc. are all separate certificates. You can have multiple category and class ratings on each. If you have a Private certificate already, you will not need to get another certificate for any further non-commercial privileges. All you need to add a different Category or Class is an endorsement showing appropriate training and a Checkride. You will retain Private Privelages for any Category and Class listed on your certificate. Glider is a separate Category to any powered aircraft.


  61.63 Additional aircraft ratings (other than for ratings at the airline transport pilot certification level).
  
  (a) General. For an additional aircraft rating on a pilot certificate, other than for an airline transport pilot certificate, a person must meet the requirements of this section appropriate to the additional aircraft rating sought.
  
  (b) Additional aircraft category rating. A person who applies to add a category rating to a pilot certificate:
  
  (1) Must complete the training and have the applicable aeronautical experience.
  
  (2) Must have a logbook or training record endorsement from an authorized instructor attesting that the person was found competent in the appropriate aeronautical knowledge areas and proficient in the appropriate areas of operation.
  
  (3) Must pass the practical test.
  
  (4) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, weight-shift-control aircraft, powered parachute, or airship rating at that pilot certificate level.
  
  (c) Additional aircraft class rating. A person who applies for an additional class rating on a pilot certificate:
  
  (1) Must have a logbook or training record endorsement from an authorized instructor attesting that the person was found competent in the appropriate aeronautical knowledge areas and proficient in the appropriate areas of operation.
  
  (2) Must pass the practical test.
  
  (3) Need not meet the specified training time requirements prescribed by this part that apply to the pilot certificate for the aircraft class rating sought; unless, the person only holds a lighter-than-air category rating with a balloon class rating and is seeking an airship class rating, then that person must receive the specified training time requirements and possess the appropriate aeronautical experience.
  
  (4) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, weight-shift-control aircraft, powered parachute, or airship rating at that pilot certificate level.


For instance, I have a Private Cert for Airplane Single Engine Land. I also have Rotorcraft Helicopter hours. I can not solo a helicopter without an endorsement from my instructor. Essentially, giving me Student Pilot privileges for helicopters without a student pilot license (minus the Class B restriction). I would need a Checkride to add Rotorcraft Helicopter to my Private Cert.