I have a command-line Python script that works well to convert one sort of file into another given a few parameters and would now like to deploy this to some of my colleagues who may not know what a command line is.
I could work for hours trying to determine which Python GUI toolkit is "best", then learning how to do what I need, but it seems like this would have been done before.  
Is there a relatively cook-book method to GUIify my program?  Direction to either some sort of lesson/tutorial or an existing, documented, concise program would be excellent.
Basically you just need to figure out what widgets will hold the data you want the best. I suspect you could use a couple combo boxes to hold different sets of extensions. Or you could just use the path name strings to figure that out. Hit a button and run the conversion process, probably in another thread so the GUI remains responsive.
I'm biased for wxPython. However, you're better off taking a look at their demos and documentation and seeing which GUI toolkit fits your brain the easiest.
Cookbook GUI interface for a command-line script