This is the modine - apparently used on an auto Fozzie - this uses the engine coolant which will go through your proposed Mishimoto unit. It has an oil feed from / to your auto box and in / out flow for coolant linked in with your engine coolant flow - Do you have something different fitted to your Fozzie?
http://www.ebay.co.uk/itm/Genuine-Subaru-WRX-05-07-Forester-XT-02-07-engine-oil-cooler-modine-EJ255-OEM-/262303904488?fits=Car+Make%3ASubaru&hash=item3d1287f2e8:g:nm0AAOSwWTRWzF2M
This is one similiar to my auto Impreza
http://www.ebay.co.uk/itm/Genuine-Subaru-Impreza-Legacy-Modine-GC8-Oil-Cooler-V1-To-V6-EJ20-EJ25-Early-/272507298369?hash=item3f72b36241:g:IpcAAOSw65FXqv9-
again it uses engine coolant to take heat from the transmission oil.
Can you post a picture of your oil cooler 'in' your radiator so I can get a view on size comparision
You could fit a separate oil cooler somewhere on the front of your motor like this (although this is designed to be connected to the engine OIL filter you could adapt it to your auto trans box
http://www.ebay.co.uk/itm/SUBARU-IMPREZA-TURBO-MOCAL-OIL-COOLER-KIT-STAINLESS-BRAIDED-HOSE-80-DEG-STAT-/351707294852?hash=item51e3636c84:g:964AAOxyOlhS2T76
One of the guys did a really looking factory job of welding in some mounts to the front of his Impreza and removed the fog light to flow air into it.
This brand does a variety of auto trans coolers (Nissens) which you could mount in the same way as the larger mocal option
http://www.ebay.co.uk/itm/Nissens-Oil-Cooler-Auto-Trans-90653-/191992938662?hash=item2cb3abc0a6:g:ZqgAAOSwux5YLdfS
Given your location consider more frequent trans oil changes - average is every 50K with less miles specified for towing / extreme climate conditions, also go for a higher quality oil than standard. If your 08 model is still on the original oil it would be a good idea to at least give it a flush (not just drain as you wont get all the old stuff out) with new stuff - expensive but worth it.