This isn't a very slick tweak, but it will get you more troops to recruit if you want them. Open your
conversation.txt file and find the following:
dlga_ms_view_officer_elements_list:ms_extra_officer_dialog_start 69631 24 6 30 2 144115188075856911 1 4 0 2122 3 72057594037927936 144115188075856910 1 2133 2 72057594037927937 144115188075856908 2108 2 72057594037927937 24 3 0 1_({reg0}_thaler_and_{reg1}_coppers). 21 1 1 2 936748722493063844 1 NO_VOICEOVER
dlga_ms_view_officer_elements_list:ms_extra_officer_dialog_start.1 69631 24 6 30 2 144115188075856911 3 4 0 2122 3 72057594037927936 144115188075856910 3 2133 2 72057594037927937 144115188075856908 2108 2 72057594037927937 24 3 0 3_({reg0}_thaler_and_{reg1}_coppers). 21 1 1 2 936748722493063844 3 NO_VOICEOVER
dlga_ms_view_officer_elements_list:ms_extra_officer_dialog_start.2 69631 24 6 30 2 144115188075856911 5 4 0 2122 3 72057594037927936 144115188075856910 5 2133 2 72057594037927937 144115188075856908 2108 2 72057594037927937 24 3 0 5_({reg0}_thaler_and_{reg1}_coppers). 21 1 1 2 936748722493063844 5 NO_VOICEOVER
These sections of code represent the 3 dialog options you normally have (ie, 1, 3, and 5). To change the first option to 100 troops, for example, change each of the red 1's to 100.
To get the dialog option to actually say 100 in game, you would need to change the title of the dialog (doesn't matter what you change it to, just as long as it's different). For example, you could change it to "dlga_ms_view_officer_elements_list:ms_extra_officer_dialog_starta" (ie, just stick an "a" at the end). If you don't change the dialog title, the tweak will still work, but when you talk to the garrison commander, it will continue to say 1 troop, even though it will actually give you 100.
That's a bit of a sloppy explanation, but hopefully you get the idea.