Is it possible/a good idea to use an ExpressCard SSD for swap?
This seems like a lot of work to me since your swap rates are easily measured and generally not at all the bottleneck that needs alleviating on OS X systems I've managed.

Losing swap will be highly destructive to data integrity so you should have your system backed up regularly and able to be restored and files reconstructed if they become corrupt during a write operation. There are several guides here on how to move swap elsewhere, but I would say it's categorically a bad idea.

If you can post back what your exact hardware model is, perhaps there are better ways to accelerate your workload. I would post about 15 minutes of vm_stat 60 and a picture of your Activity Monitor memory when the system is fullly loaded and stable running your desired workload.

My initial thoughts are that you should install your OS onto the SSD and have the home folder on the normal "boot drive" rather than messing with swap. Apps will run properly, you can remove just about all installed apps from /Volumes/SSD/Applications and let the Library there and core OS have the entire space for swap and OS. Moving some highly used data files there might make the most sense as well. Again, these are educated guesses and without knowing the CPU / RAM / internals of your specific Mac, there might be better solutions as well.