Once it's out I'm sure people will make various mods (like BF2 and Halflife - think Foxbot) including playing single-player in multiplayer maps.
That "IDK" threw me for a loop (I don't know...der) - SDK would be my answer - Crytek released the tools needed to mod Fary Cry back in the day, and I'm sure they will do the same for Crysis.