In MED:
Edit -> Transform model global -> Center model

Alternative: select everything and just move it for an off-center origin

In Realtime with lite-c:
Move every Vertex in the opposite direction

Example Code:
Code:
void ent_movevertex(ENTITY* ent, VECTOR* pos, int num){
	
	CONTACT* c = ent_getvertex(ent, NULL, num);
	c->v.x = pos->x;
	c->v.y = pos->z;
	c->v.z = pos->y;
	ent_setvertex(ent, c, num);
}

void moveOriginBy(ENTITY* ent, VECTOR* offset){
	int i;
	VECTOR move;
	vec_set(move, offset);
	vec_inverse(move);
	int vNum = ent_vertices(ent);
	for(i=0;i<vNum;i++){
		ent_movevertex(ent, move, i+1);
	}
}



Note: this will change every entity with the same model file - to avoid that use: ent_clone