You should be able to add it
In pcgen, when creating a character that has the Maneuver Master archetype, on the feat menu, the bottom left selection allows a special "Maneuver Master Bonus Feat" which has all the improved maneuver feats available:
This allows you to add any improved maneuver feat as a monk bonus feat from level 1.
Possible fix for 6.08
After testing with version 6.08 I confirmed that the option is unavailable in that version, while in version 6.07.08 that I have the options works. One way to try to fix it is to use the option from the previous working version.
I tested this myself and the option worked, however, I will stress that messing with pcgen files may have unintended consequences so use this at your own risk. What I did was go to the pcgen installation folder and find the following file:
You should backup this file just in case something goes wrong. Then, open the file with a text editor and find line 946, which should start with:
Bonus Feat KEY:Maneuver Master ~ Bonus Feat
Then replace the entire line with:
Bonus Feat KEY:Maneuver Master ~ Bonus Feat CATEGORY:Special Ability TYPE:MonkClassFeatures.ArchetypeAbility.SpecialQuality DEFINE:MonkBonusFeatLVL|0 DESC:In addition to normal monk bonus feats, a maneuver master may select any Improved combat maneuver feat (such as Improved Overrun) as a bonus feat. At 6th level and above, he may select any Greater combat maneuver feat (such as Greater Grapple) as a bonus feat. At 10th level and above, he may select any maneuver Strike feat (such as Tripping Strike) as a bonus feat. BONUS:ABILITYPOOL|Maneuver Master Bonus Feat|1+MAX((MonkBonusFeatLVL+2)/4,0) BONUS:VAR|MonkBonusFeatLVL|MonkLVL SOURCEPAGE:p.58
Upon restarting pcgen, the option to select a maneuver master feat should now be available. However, do note that the option is bugged (even in my version) since it grants 2 bonus feats instead of one. This is probably just a minor inconvencience, though, remember to pick only 1 of these bonus feats.