var radius = 32;

function part_circle(PARTICLE* p) {

if(p.lifespan < 100 && p.skill_a == 1) {
p.skill_b += 5*time_step;
if(p.skill_b > 360) { p.skill_a = 0;}
else {
p.x = p.skill_c+radius*cos(p.skill_b);
p.y = p.skill_d+radius*sin(p.skill_b);
}
}

else {
vec_add(p.x,vector(p.vel_x*time_step,p.vel_y*time_step,p.vel_z*time_step));
p.skill_c = p.x;
p.skill_d = p.y;
}

}

function part_circle(PARTICLE* p) {
p.vel_x = ...
// do NOT (!) set p's MOVE flag
...
p.lifespan = 160;
p.skill_a = 1;
p.event = part_circle_move;
}

Untested.