Yes of course you can do this if you have a midi interface with an output.
If you use the same keyboard as your controller then you'll need to switch "local off" in it's midi settings.
Cubase "External Instruments" is just a feature that I guess is trying to make hardware seem like a part of the software...it does not disallow the use of actual hardware external instruments in any way.
Just means they don't integrate in the same way as with full versions External Instruments set up.
i7 firstname.lastname@example.org, 12Gb, W7HP x64, Cubase 7.5.20 x64, Steinberg UR28M, Alphatrack, Wavelab Elements 7.2.1.