I'm not sure if the new style key with the buttons built in it are compatible with the old style key / fob combination. I have purchased two new fobs off ebay for my Expy and they came with instructions. Like was mentioned, you have to program them at the same time because the old fob signal will no longer work unless you reprogram it. Since it worked, I bought extras for all my other cars too.
I just tried an extra transponder key I have for my 09 Lincoln (same as for all newer Fords) but it doesn't fit all the way into the ignition of the Expy. The key cut is the same, but the plastic part with the buttons is too thick to fit into the shroud that surrounds the ignition switch. The Expy keys are thinner, so they fit all the way in. I don't think your plan would work even if the electronics were compatible.