I do not advise you to do that, as any updates that happen in the future will OVERWRITE the modified file and you then lose all the modifications you have made.
it is easier to just put the galaxy's in the galaxyforge folder in the HIDDEN loacal appdata folder which is the SAME path as mods.
BUT if you really MUST edit the galaxy.manifest file the points to note are
1 the number of maps at the TOP of the manifest (add the number of maps you are adding to the count here)
and
2 the exact format of the lines for EACH of the maps (same leading part("FileName ", with the map file name in quotes(without extension (the bit after the full stop(.))) also note that the file IS VERY CASE sEnSaTiVe do NOT alter the case if you want it to work (add more lines with the 'Filename " ' your maps name ' " ' for each of the maps you are adding
3 blank line at the end of the file.
back at sins 1.15/entrenchment 1.00 release there was such an issue where the maps were not relocated yet and were needed to be added to the manifest, and I wrote a small utility to replace the galaxy.manifest with one that listed all the files in the program files galaxy folder, (this is in my modding utility pack)
hope this helps
harpo