would it be possible to...

[Sfwartir]

Yup x 2, no problem:

1.Recruitable mercs:
Add the faction you want to be able to produce the unit to the ownership line of the actual unit in export_descr_unit.txt, add a recruitment line (or multiple lines) in export_descr_building.txt.
The unit will use the mercenary (green uniform) texture in descr_model_battle.txt, no matter which faction recruits it though, unless you change that too.

2.Produce all units in one turn:
Open export_descr_unit.txt, find the stat_cost entry for all units, and change the first number after the phrase "stat_cost" to 1.

E.g if the entry is

...
stat_cost 2, 150, 100, 20, 30, 150
ownership britons
...

change it to:

...
stat_cost 1, 150, 100, 20, 30, 150
ownership britons
...

There you go.