If you plan to do a manual upload anyways would it maybe be a solution to mount the import folder also to your Nextcloud instance as an external source, of course if you have Nextcloud...
If you do it like this you could easily use Nextcloud for uploading files manually.
From what I know uploading photos is only possible to one directory and that is the import folder, however you can index multiple directories but that is a different thing because they are only indexed and not imported.
Maybe try to open up a Issue on the official GitHub from Photoprism here to get a answer from the developers, if it's possible to import from multiple sources please let me also know: Click