vec_rotate. search for it in the manual. when you know how big you want the force to be, make that the x-component of the vector, with the y- and z- equal to 0. vec_rotate by the angle, and then you have that force in that angle. then use ph_addvelcentral or something similar.

i'm assuming this will answer your question -- if you just want to add torque then MrCode has your answer ^^

julz