Can fat grafting be used to add fullness/create cleavage? How soon can it be done after implant surgery?

Fat grafting to improve the cleavage area can definitely make a big difference, but I have not had a great deal of success in drastically improving the projection of the breast after implant based reconstruction. That is one of the main drawbacks of implant based reconstruction thus far that has been noted. Things that can improve the impression of projection is nipple reconstruction, but that again is not for everyone. You already have high profile implants so that tool has been utilized already. I typically wait at least 3 months after implants are placed to pursue fat grating, as this gives the implant enough time to settle into place so you can see where fat grafting would be most beneficial. Recovery from fat grafting is fairly quick as there are no large incisions, but do know that you may be bruised/swollen for at least 1-2 weeks after surgery and the swelling where the fat is injected and harvested from may persist for up to 3-6 months. 

Yes, fat grafting is great for exactly what you're describing and can be done at any time.  Recovery tends to be relatively easy. Discuss with your surgeon about exact timing.

Sometimes fat injection is performed as an ancillary procedure for things that you mentioned.It can help with deficiencies and irregularities.