My question is regarding the PILOT(X) keyword.

If you pay a pilot's printed cost to play it as a unit, can you then pay its pilot cost to play it as an enhancement on a valid vehicle unit?

The PILOT(X) text doesn't seem to specify that it can only applied only from your hand, and the rules in the Rogue Squadron boosters also do not seem to specify if this is or isn't possibly either, so is this possible?

No. You either play it as a Unit or as a Pilot/Enhancement (afaik)

The Pilot keyword allows you to play a unit from hand as an enhancement or have it enter play as an enhancement. It does not allow you to have a unit in play with the Pilot keyword become an enhancement.