Depending upon the type of compression there is also the cheating bastard approach to dodging it when hacking.
If the compression format has an uncompressed marker (or in some cases could just put one lot of [string]) then make the uncompressed version and then insert the uncompressed marker however many bytes apart it is for that format.
Sure finding the call for the compressed read* and then changing it to a direct read/dma/whatever is the better method by far, as is making your own recompression tool, but there are times when playing the cowboy works just fine. Granted on older systems that is not as common as the likes of the GBA or something where higher level compression formats were more of an option.
*Sometimes you get lucky and there is a table/flag that says what things are compressed or some kind of autodetection, twiddle the table/flag or nerf the detection and you are good to go.