If you want to do this, it's probably the easiest thing about putting in the BFM - splice the three wires coming out of the BFM dimmer circuit to the corresponding three wires of the dimmer switch. You have to disconnect the dimmer switch from the regular subaru illumination module so you won't be able to dim any of the other lights anymore.

Honestly, 5 minutes with the wiring diagrams available on this site will show you want to do.