It definitely can be done. But isn't easy.
The first question is whether you were competitive for a HO fellowship prior to your NHSC service. Will depend upon whether you're an MD/DO/IMG, went to a Univ or Comm based residency, USMLE scores and attempts, and any research you might have done as part of your residency. Also will depend upon whether you have good LORs from HO docs, and whether your residency institution had a HO fellowship.
In any case, you'll probably need to spend some time working to gain recent HO experience.