The manufacturer says that your serial adapter does not support Localtalk networking. (I don't *think* there are any USB converters that support Localtalk, but someone might correct me on that. Supporting RS-422 isn't enough by itself to do it.) So there's no way you're going to be able to use the normal "chooser" method to network those two.

The simplest solutions to your quandary is either an Ethernet card for the SE/30 (yeah, I know, not easy), one of those Localtalk->Ethernet converter boxes, or an older Beige Mac that has both sorts of ports to act as a go-between. The only other choice I can think of offhand is to either use a serial comm program on both systems to transfer files, or set up a SLIP/PPP TCP/IP connection. (Highly nontrivial.)