@Xagoln:
It might be sufficient to decrease the version numbers of libc6 and libgcc1 in the dependencies of Debian Wheezy's opus-tools to get it running under Fremantle. All other dependencies are either met by Fremantle or can be installed directly from Debian.

If that doesn't work you could use opus-tools via Easy Debian (e.g. Estel's Wheezy image).

Edit:
I just tried to get Wheezy's opus-tools running natively on my N900 but both the encoder and the decoder didn't run due to unmet library symbols.
So just tinkering with the control files and installing the Debian packages isn't enough. It will have to be compiled properly.
But given how long transcoding takes even on my Desktop computer I wouldn't want to do this on my N900.