As far as i'm aware you can't on a 5mx. Although Compact Flash wifi cards are available they are the wrong type (there's type 1 and type 2, can't remember which one the Psion supports but wifi cards are usually the other type) and I think they draw too much current. You can buy rs232 to wifi adaptors intended for enabling embedded rs232 equipment (things like cash registers or bar code scanners) with wifi. I don't see why one of these won't work on a Psion, but I'm not sure how small they are. You would also still only be limited to rs232 speeds.
I have used my mobile phone with my Psion (in EPOC, but I see no reason why it won't work in Linux) over infrared and serial and it worked fine. Any phone with IR support or a serial data cable should work. I've sucessfully used a Nokia 6230, 6280 (which supports 3g) and Siemens S25 and ME-45 phones. Unfortunately serial and IR support seems to be disappearing from modern phones in favour of bluetooth and USB. Somebody did post a hack to use a bluetooth module with a Psion ages ago or you can now get serial bluetooth modules like these - http://www.active-robots.com/products/radio-solutions/bluetooth-serial.shtml