Not in my campaign, as I don't allow simulacrums to have their own simulacrums.
That said, I see no reason why you couldn't True Polymorph a simulacrum, but that wouldn't make it any more real than it already is. In other words, it would still be susceptible to being dispelled. While polymorphed, however, the simulacrum would be able to function as a living creature in all respects (assuming you polymorphed it into a living creature), including recovering the hit points of its assumed form. However, it would still not be able to heal its simulacrum hp, which are separate.