How are you trying to create this mod, text files or module system? I believe mercenaries have to be listed between "trp_watchman" and "trp_mercenaries_end" in the troops file. If you are trying to edit the text files directly (or are using my troop editor) then it easy to modify existing mercenaries. However, if you try to edit the text file and add a new mercenary unit in the middle then this will mess up the troop id that is used in all the other files so this won't work. Therefore, if you are trying to add additonal mercenaries then this can really only be done by using the official module system. My suggestion would be to wait until the 1.003 module system is released and then its pretty easy to add as many mercenaries as you want, etc.