On Effexor (venlafaxine) XR 75mg, Can my doctor add Wellbutrin to help with depression and also the sexual side effects of Effexor (venlafaxine)? Dosage?

They can. but a plan is needed and pills don't fix all problems. Remember that ! See if you can find what your issues are and find a good talk therapist.
Not likely. In my experience switching entirely to or adding Welbutrin seldom if ever improved the depression and rarely improved the decreased libido. The depression usually responded to increased dosing of antidepressants but the decreased libido was always a problem.