[Zubersoft]

MobileSheets requires offline access to all of the files in the library. Once you import file, it will be copied to the storage location listed under Settings->Storage (unless you uncheck "Let MobileSheets Manage my Files". If you move the files under the storage location, it will break MobileSheet's' ability to access those files as it won't know where you've moved them. So no, you can't move the files to Dropbox without it causing failures in MobileSheets. 

Mike