Hi
I have a old unit converter program I did back them for symbian, Qt4 widget based, not sure if it will work for N900.
You can add new units very easily without any programming skills so you such be good to add imperial to metric.

Check the site soft-ingenium dot planetaclix dot pt/yuc/Yuc dot html and the code is (it seems I cant post links because I'm a forum newbie so to get the code go to github, username joaodeus and repo is named yuc)

Hope it helps