Hey Tim!
Looks like there is a feature request for this here: Prompt for a file at Runtime so you can add your vote to that.
The only way to get around this now, would be to import the file via the DropBox Import, and then when you want to change the file, go into DropBox, and replace the file with the new one. If you try to add a file that has the exact same name, DropBox will prompt asking if you want to update the existing file. This will allow Parabola to keep the connection to that file.