I thought it was possible trough the modem, you could directly connect the PC-modem and 5200-modem with each other, i'm not really sure about this. You should check your Computer/OS help files on this, I think it's mentioned somewhere, I can't check this because I don't have access to my old 5300 at the moment and I only have OSX here so I can't check the OS9 help files.
hope this helps a bit.
-Vanquish