If all you want to do is be able to use seek you could try this media player: Foobar2000 It's free and will let you use the seek function with unseekable SHNs (not all SHNs are unseekable though, just some). It doesn't instantly seek like with FLAC or seekable SHNs, but it's pretty fast. You can find the shorten (SHN) decoder plugin for foobar on this page: http://foosion.foobar2000.org/components/

I'm not against converting SHNs to FLAC, though. There are a lot of home server type players that I think can't be rockboxed so will never support SHN, but they will play FLAC.