But if I could send files through a phone-line cable (from the iMac to the SE/30), that would be such a time saver.
The only problem is, how do I?
I do have this small box with phone jack plugs, which attaches to the SE.
Any tips on this would be greatly appreciated!
Assuming your iMac has a built-in modem, if you get another modem for the SE/30 you can adapt the instructions below. Any external PC modem will work, you just need the db-25 to din Mac cable. Note that modem communications will not work with the little Localtalk box you've got.
http://support.apple.com/kb/TA21796
That's probably the least expensive option. Asante Localtalk to Ethernet bridges show up on ebay regularly and you might even find a scsi to ethernet adapter. Expect both those options to be expensive, though less expensive and rare compared to an actual ethernet card for the SE/30.