I've got a serial-to-USB adapter, and NCX [0] by Simon Bell is a fully-featured device manager for importing and exporting documents, installing packages etc. It's basically a modern replacement for Apple's old Newton Connection Utilities.
In addition to the expansion slot, the eMate also featured a single non-CardBus PCMCIA slot. It could be used for a number of different cards, including modems, ethernet cards, wireless cards, bluetooth cards, and flash memory (linear and ATA/Compact Flash).
[0] https://newtonresearch.org/connection/index.html