How do I add in an already existing GPA into my cumulative GPA?
A cumulative GPA is the final GPA from a single university, college, or school, not from all of them. 

While it may be tempting to "raise" the GPA from a particular university by incorporating better results from another school, you will also be lowering the GPA from the school where you did better. 

Admissions officers or committees are interested in how you did at each separate school, particularly if you can show increasing cumulative GPAs over time.


  Generally, students' computation or renormalization of their own GPAs
  is viewed as at-best-irrelevant by most interested parties, or, worse,
  as deception or manipulation. Let your transcripts speak for
  themselves. - paul garrett