Huh? There are no federal officers charged with crimes to be expunged.
Think back to neighboring Idaho, and what happened to the murder charges against FBI womankiller Lon Horiuchi. But since no trial took place, charges for either the murder or for the deprivation of the victim's civil rights can still take place. And as we've seen of late, such charges can be brought even thirty or forty years after the fact.
Too, you'd do well to check out the following FReeppost. Should NooYawk politicians try to infringe upon the rights of Wyomingites, I don't think they'll be at all happy about the results.