Either my search-fu is sorely lacking or this is undefined. Any thoughts?
I don't believe the rules are that specific on the matter. As a houserule I suggest either looking at the hardness and hit points of the door or surrounding wood- or stone-work and being able to do a point of damage to each with an improvised weapon (the piton and hammer) or looking at the Disable Device skill (spiking a door would probably fall between jamming a lock with a time of 1 round and sabotaging a wagon wheel with a time of 1d4 rounds).
If done while rounds are being counted, I would say retrieving a hammer or piton is a move action, of course, and hammering one in under 'normal circumstances' would be a standard action. If the dm thinks its going to be a little tougher to pull off, might as well use CMB or possibly hardness. I've always done this stuff on the fly.