Pyro vel Render Pass

[venator11387]

I recently created a fireball using pyro, I want to render it out, and apply a vector blur in Nuke instead of rendering with motion blur. I have looked at some other methods online, including this forum, on exporting the vel pass. However, they all involve using a shader and I already have a pyro shader assigned to it. I'm inexperienced in using mantra vex variables, so does anyone have any solutions for this? Thanks.