I've used black peppercorns (unground) to the boil. About a teaspoon on a 5gal batch, 5' before flameout.
What pepper does in the boil, as far as I can tell, is deepen and round all other flavours, whilst contributing very little flavour of its own.
If it's pepper flavour (and the accompabning heat-kick) you want, I recommend dry spicing.
Knockout...I dunno. The yeast will do stuff to whatever's in your wort, and I've no idea how it'll treat the pepper.