The Drive by Wire (DBW) to Drive by Cable (DBC) swap is possible to do from a wiring and mechanical perspective but there are a few things to consider first.
The DBC engine being swapped in is likely an engine from an older model than the car being swapped like an 2007 RS to 2005 WRX so it would automatically fail an inspection. Although your state may not currently check for this more states (Utah for example) are adopting this policy so before you do this swap, you should consider something newer to ensure your car won't become unregisterable. The exception would be a 2005 non turbo car that came with DBW while the 2005 WRX is DBC. This conversion could pass smog because it just needs to be at minimum the same year.
2. Cruise Control
From a wiring perspective, the biggest issue is that cruise control will be lost. This is because the stock DBW ECU will be removed and it was in charge of the cruise feature. With any NA to Turbo swap, the ECU that matches the new engine gets wired up and the original is removed as a byproduct.
3. Pedal Mounting
To install a DBC pedal into a DBW chassis, a custom bracket needs to be fabricated. Please note we have a bracket for DBC to DBW conversion but this bracket will not work the other in this application.
OTHER SIDE NOTE FOR THESE SWAPS
Starting in approximately 2006, Subaru switched to a return-less fuel system on the NA models. This means that for a turbo engine swap the fuel tank hanger assembly needs to be swapped out for one with a return line and then that line will need to be run to the engine.