Both FK and IK are the matter of rigging and rig positioning on key frames
via 3D editor which is further converted to Valve compatible SMD, holding relative sceleton positions per each individual frame which in total binded to a single animation sequence name after compilation to MDL. So, the sequence name of model, frame number and the frame rate are the only animation related things available for manipulation in game engine through sourcepawn. There are also animation layers / poses / activities, accessible through sdkcall to CBaseAnimating & CBaseAnimatingOverlay (see:
animhelpers). However, under the hood they consists of same pre-defined coordinates without ability to create your own animation dynamically if that's what you asked.