I just looked at the source code. It's actually quite simple, just a standard LZSS compression. I'm sure there's source code for LZSS compressors out there. You could take a look at one and adjust it to yield the compressed output in SLZ format. Just try it, I'm sure it ends up being easier than you had thought.
If you can't program, then you're probably out of luck, though. You could perhaps ask CUE (he's on this forum as well) who seems to be the author of the decompressor, maybe he has a matching compressor.
The format also has an uncompressed mode (named "0/STORE" in the source code). If there's enough space you could just reinsert everything uncompressed.