It's not supported to add media from a client.
You can sort of work around it by using UNC / mapped paths that the server can see, but this limits you to adding content on a LAN (so not from outside your LAN).
I posted about this earlier, actually, or intended to anyway... This
used to be a simple workaround, but it doesn't work correctly right now (or not as ideally as it used to work).
Right now to do this:
MC18 won't let you rip files when your copy of MC is in Client Mode
at all. Ripping is totally disabled. What you have to do is switch to a local library, and then rip the files using the local Library. However, if you rip them with that local Library copy of MC directly to a network share that is watched by the Server (or rip them to a local folder and move them, but you might as well go directly because that works just as well), it'll automatically find the new files and import them. When you are done ripping, just switch back to the network copy of the Library, and they'll be there waiting for you.
You can put a UNC path (or a network drive location) directly in the Options > File Location > Audio path field and it'll rip there. Just make sure it rips somewhere the Server's copy of Auto-Import is watching.
But before:
You could just set the Options > File Location > Audio Path to the network drive that the server was watching and it would let you rip. The problem was that it would let you rip no matter what (so it would lose the files if you ripped to a non-watched location). That's bad and it led to confused customers for sure. But the solution was to just disable ripping entirely when you are connected to a server. So, now you are forced to switch to a local Library (because otherwise the Ripping option is just completely disabled). MC got both "smarter" (by preventing people from trying to rip, only to have MC lose the files) and "worse" (because now you have to switch to a different library first, which is annoying if you want to rip some discs while you're doing something else in MC at the same time).
I get it, but it would be nice if we could find some sort of middle-ground.