The switch for the spoiler on non-SRT XF's will NOT work. It is designed for very low current, and functions as a signal source to the BCM.
The BCM has the large transistor drivers that actually power the spoiler.
You would need to design a circuit with relays that would be able to supply voltage and the required amperage to the spoiler, and be able to switch polarity.
This could also be done electronically with some op-amps and power transistors, but the relays would be much simpler.
Another way to do it would be to adapt an old style window switch that flips polarity.

The main problem with all the above is that you would not be using the limit switches within the spoiler circuitry, this would probably result in the mechanism locking itself up at times.

An electronics expert ( not me ) could probably design a circuit that would take the feedback from the limit switches along with input from an up/down switch to accomplish the job.
This would be a Rudy Compart kinda thing, like the one touch convertible top mod.

All that being said, good luck with the project !