Today I had some time to look into this and unfortunately it is not what I was looking for... but your answer made me define more precisely what I am looking for.

So I made a custom IM status named "Test" (with facebook online, sip offline and skype online). What I am looking for is the file that defines that status ; the file where I should find those IM networks defined (with facebook and skype online and sip offline) so I could customize the IM status a little more by editing directly the file.

So the question becomes : where does maemo store the custom IM statuses files?