If you mean music from a normal cd then rest assured you CAN convert WMA to MP3 quite easily, see below. However, if you are referring to DRM music downloads (the licensed types) in WMA format this will not work. Its frustrating I know but you need a license key and compatible hardware to do the latter, not all Iriver models are currently compatible.
I also have an IRiver (the H140 model) and quite frequently convert cd music from WMA to MP3 in order to edit and mix music and save on my IRiver disk space.
The programme I use is:
dBpowerAmp Music Converter 11.0 pop
Cut and paste this exactly in to the search box of a site called download.com.