How do you open and get a IFR clearance while in the air?
There is one example in the AIM used for transitioning from VFR to IFR, and it how I've always done it:


  AIM 5-2-5. Abbreviated IFR Departure Clearance (Cleared &hellip; as Filed) Procedures
  
  EXAMPLE-
  "Los Angeles center, Apache Six One Papa, VFR estimating Paso Robles VOR at three two, one thousand five hundred,
  request IFR to Bakersfield."


Reference: 5−2−5. Abbreviated IFR Departure Clearance (Cleared &hellip; as Filed) Procedures (13 MB PDF)

Notice that they specifically say "VFR" right before their current location and also use the word "request" to make it clear what they are looking for.  I would change your request to:


  Tampa Approach, Cirrus 123AB, VFR 5 miles southeast of Tampa Exec
  at 1000 feet, request IFR to Ft. Lauderdale Exec