You're allowed to use Errant's guns in whatever you want - he said so himself on another thread, so long as you got them legally...
Guns aren't hard coded which allows much more freedom, but makes it harder to get one working. You'll need to look up tutorials on editing an existing script into one you can use for Errant's weapons, if and when you do get to that stage - look up GeoDav, he's got great tutorials...
Quote: "You would have to first develop a layer over UDK that emulates FPSC's functionality, and then build upon that framework. It won't be as simple as going from say DarkBasic Pro to Blitz Basic 3D, since the two development mediums are extremely different."
I disagree, as far as scripting, you're right. But in the editor, it's essentially FPSC with more functions and features.