The mission DLLs are compiled executable files. The text can be modified but not easily (a bit of programming knowledge is required).
For the missions that we have the source code for (in the case of Plymouth Cold War and many of the multiplayer missions) they can just be recompiled with the new text strings.
It might be too time consuming however to replace the text in the multiplayer missions. (Checksum busting data would have to be added to the end of the files there as mentioned for the techtrees). The only problem with that is, if the file is modified in any way by automated tools or whatnot (for example, changing the resources stored inside the file), the checksum will be changed, and the data at the end could be wiped out.
As far as the mapper goes, extracting multiple files at a time is buggy. If you need to extract files, do it one at a time or use the VOLextractor (best method).
Neither tool can decompress the data, only extract it in a compressed form. As Hooman said, we can decompress the data (though it is very rough... the filenames are hardcoded, and garbage is output at the end of the file (not our fault, it's a side-effect of the compression algorithm. Since the game doesn't store the original uncompressed size of the file, it is hard to automatically remove the garbage. When the game decompresses a file, the code that reads the techtree / spreadsheet data will just stop when it encounters the garbage data (treating it as invalid / erroneous data)).
