Hey Erik,
I gave you the pieces to construct a real tool. 
You can concatenate variables, so this sort of thing is relatively simple.
There are various pitfalls to watch out for when assembling shell scripts.
KM user-input is takes a bit of experimentation, before you understand its subtleties.
In the macro below I've tried to demonstrate all of the various input forms. If your Dropbox folder is in the default location it should just work.
* Note: Keyboard Maestro has a Custom HTML Prompt action, so if you know HTML, CSS, and JavaScript you can get really fancy.
-Chris
