Yes
Sp can be created with ut2k4.
But I suppose I depends on gametype, if you wanted a 'run round corner and shoot guard' senario then I suggest using the xPawn as the Bots (NPC's) and trigger them using the scripted sequences..I have been using that for ut2k4 SP.Even though the NPC's are AI unfriendly,play around with the available commands on the scripting 'list' (WaitForEvent,TurnTowardsPLayer,FireWeapon etc...) and a reasonable balance can be created.
And as Sarevok said, many triggers needed
I hope that can help you, I would like to say a lot more for SP ut2k4 and if you require help for NPC action(s) them pm me and I should be able to help.
Lastword is,if you use the Monster xPawns as 'enemy' (Skaarj......etc) then that could be just another 'MonsterHunt' game and maybe no fun,but that depends or your SP ideas.