Can you increase your undergrad gpa after graduating?
Yes, it is possible.

Faculty at my university have the right to change their course grades up to five years after the course ends.  I have a standing offer that if any student publishes a research paper as a result of work in my class, I will retroactively change their grade in that class to an A+.  I have actually done this a few times, in one case two full years after the course ended.

(All the students involved in those changes already had A's, and my university counts both A and A+ as 4.0 in its GPA calculations, so the change did not actually change their GPA.  But in principle, it could have.)

But this is the only way I can imagine a student at my university changing their GPA after graduating.  In particular, I believe the university maintains separate GPAs for different degree programs, so taking any post-bacc or graduate courses would not effect their bachelor GPA.