I believe that they can return as long as they don’t sign with an agent and pull their name before the draft. I don’t know if participating in the combine would be enough on its own to end the eligibility clock.
Everyone who is considering going should have a draft grade by now and you only have until the end of the bowl season/playoffs to declare. Anything after that would force you into the supplemental draft.
It would be rare but I could see someone who still had eligibility that perhaps got hurt prepping for the draft pull their name out and come back to prove themselves.