It is possible. The size would not be suuuuch an issue, since an implant can be removed relatively easily. The problem would be that if your breasts have a growth spurt after you've had an augmentation, fat and glandular tissue and such probably won't react very well to the implant. It's probably going to either look weird, either halt development, or some other consequences. I think it would be quite a big deal if that happened.
And well, you don't say you have "small breasts", but rather undeveloped breasts. I think that even if you're destined to having super small breasts, they should still reach at least Tanner stage 4. Even the older, unlucky ones with especially incomplete development generally reach stage 3 or 4.
A small lump of fat as you describe it is not a small stage 4 or 5; it's a much earlier level of development. I think you're due to have more development, even if you need a change in your HRT regimen for it.
I've read a few stories in the past of having a surprising, sudden growth spurt in the breasts after months of stalling or very slow development.
So, yeah, wait until your breasts have had a stable, more uniform shape for many months, at least, I guess, before deciding on breast augmentation.