Will another HT to add density where native hair has fallen damage existing implants and what is donor area limit?

If you are considering a cosmetic surgery it would be in your interest to see a doctor for an examination to make sure you would be a candidate.  Second surgery should not damage your transplanted hairs.

A great first step would be an online consultation. Often you can submit photos to top hair restoration surgeons for a complimentary evaluation. This should help lead you in the right direction. Having multiple consultations with different doctors cannot hurt. 

In the hands of a good surgeon, you can have more grafts added to thicken up your previous hair transplants

Thank you for your question, It is difficult to advice you without looking at your pictures but, if you want to fill the gaps it can be done, an experienced plastic surgeon will be able to plant hair parallelly between the two hair follicles successfully. A good donor area has 5 to 6 thousand hair grafts that can be extracted in 4 sessions. Each time we extract the no. of grafts will be lesser than the previous session. You can consider doing the procedure to fill the transplanted hair area as well as your crown.