Ingo R.

Having support in F-Stop itself would be nice, but as workaround, I can recommend FolderSync: That easily lets you sync files with your NAS.
What I do is I create a sync in FolderSync for the pictures I want to tag, sync them to my phone, then do what needs to be done with F-Stop, and after that sync back to the NAS with FolderSync. Of course you need the space on your phone, but if you don’t have external SD card in your phone, you could always just sync the subset of your picture collection that you want to tag next. FolderSync has a lot of filters for file and folder names.