This really depends on what happens during the transfer. If the Pokemon's Contest Condition is kept during a Pal Park transfer, then this might be an issue. If the Contest Condition resets, (which it probably won't since everything about the Pokemon, including ribbons, stats, and held items, should remain the way they were) then you'll need to raise your Pokemon's Contest Conditions from scratch, so in this case you'd be able to feed your Pokemon poffins.
The main factor that indicates whether or not a Pokemon can eat more Pokeblocks/Poffins is Sheen, or fullness. You can indicate the "Sheen" of a Pokemon by looking at how many stars are encircling it on the contest stats screen in R/S/E, or the "Sheen" bar in the Summary Screen of D/P/Pt.
So bottom line:
If your Pokemon's Sheen bar is full, it can't be fed anymore Poffins or Pokeblocks.
Hope I helped!