A: Although the attachment of attaching Creepers is almost instantaneous (happening even before something like the Elder Sign or Necronomicon can protect itself from the Creeper) the intent of Surprises is to nullify the targeted card play as if it hadn’t happened, or, in the case of the Keeper-nullifier, as though you had played the Keeper directly to yourself instead of your opponent playing it to themselves.

“Out of Turn: When another player plays a Keeper, it goes in front of you instead of them…”

So, in this case, it’s as though the Keeper being hijacked would technically not hit the table in front of your opponent at all, and therefore it is not possible for any of their Creepers to attach (or be covered, in the case of the Elder Sign).

TLDR: If you Surprise a Keeper-play of someone with a Creeper that could accompany the Keeper, you are not forced to take the Creeper. You just get the Keeper.