There are kits for the E46 that fit our cars perfectly. All you need to do is to buy them, and tap into your oem transmission oil cooler lines (heat exchanger). I looked into this a while back, but felt it wasn't really neccesary, since your water temps mirror your trans fluid temps because of the heat exchaner. Good Luck, and let us know how it goes.
Cobb AP, K&N, AA Catless DP, AA Custom Hybrid Axlebacks, ETS 5" FMIC, BMS dp fix V3, Meyle HD; Tie Rod Assembly, Control Arms, Fr/Rr Swaybar End Links. Koni Yellow Shocks, Megan Race Strut Bar, Wavetrac LSD, Ar design Oil cooler, 1M Water Pump, Low Temp Engine Thermostat, Zimmerman Cross drilled Rotors, Textar Brake Pads, Pilot Sport Cup Tires, Valentine 1