How to convert L609 4wire to L6 3wire
There is NO neutral in a NEMA L6-?? circuit. That is a straight 240V circuit/receptacle.

I have no idea what an "L609" is. There is no such NEMA number, but you CANNOT simply run a loose or separate neutral for this circuit. It MUST be run with the circuit conductors in the same cable or conduit/raceway. It must also be correctly sized based on the required circuit and overcurrent protection.

Also, what is the cabinet you speak of? Is this what you are feeding or where the feed comes from??