Those look like adapters to run two network connections through a single Cat-5 cable... Two connections go in one end, a single cable connects the two, and two network connections come out the other end.
They make these adapters so that you can run two devices on a single (presumably already run) network cable... It works because in a standard 100mb or 10mb network connection you only actually use two pairs (out of 4 pairs in a cable), so one of the incoming network connections is switched over to the unused pairs of the single existing cat-5 cable.
These won't work with Power Over Ethernet, or Gigabit Ethernet.
But no... unfortunately you CAN'T use them as you stated.
I'll second Admiral Michaels advice: Wrt54GL (Or V4 or earlier Wrt54G) with custom firmware.
I use Talisman from Sveasoft... lots of options, easy to setup and configure. Not sure where the other firmwares are at these days... haven't looked into them for some time.