Don't worry about the don't modify by hand bit - just make sure you're doing it in a plain text editor like Notepad - not something likely to try and change formatting like Word.

You need to follow format of what is there quite exactly, curly brackets, tabs, spaces and commas all have specific meaning to game and will cause CTD's if not placed correctly.

In export_descr_units, you just need to change ownership lines, you don't need to worry about removing existing numdian units unless you want to play custom battles. Find the units you want Bactrians to recruit ie.
type east infantry
dictionary east_infantry ; Eastern Infantry
category infantry
class spearmen
voice_type Light_1
soldier persian_sparabara, 60, 0, 1
attributes sea_faring, hide_forest, can_sap
formation 1, 1, 2, 2, 5, square
stat_health 1, 0
stat_pri 3, 1, no, 0, 0, melee, simple, piercing, spear, 25 ,0.73
stat_pri_attr spear, spear_bonus_8
stat_sec 0, 0, no, 0, 0, no, no, no, none, 25 ,1
stat_sec_attr no
stat_pri_armour 3, 2, 5, leather
stat_sec_armour 0, 0, flesh
stat_heat 0
stat_ground 2, 1, 0, -1
stat_mental 2, low, trained
stat_charge_dist 30
stat_fire_delay 0
stat_food 60, 300
stat_cost 1, 330, 150, 40, 50, 330
ownership eastern, slave
And change last line to add Numidia
ie. ownership eastern, numidia, slave

That line can contain either culture or faction - if you were going to use eastern culture you'd just leave it as it is - but I think you said you were using greek, so unless greek is on that line you need to add numidia.

in export_descr_buildings
first search through for any recruitment lines that have numidia in curly brackets i.e.
recruit "numidian desert warriors" 0 requires factions { numidia, }
if they are the only faction in the brackets comment out the entire line by putting ; before recruit ie
;recruit "numidian desert warriors" 0 requires factions { numidia, }
if another faction is in the brackets delete numidia, so its left as requires factions { carthage, }etc.
Then find the units you want them to be able to recruit and add numidia,
ie.
recruit "east infantry" 0 requires factions { eastern, numidia,}
the line used in quotes is the type line from export_descr_units which you should have already found.

Unit recruitment does get tied up with which culture you're using so it might be better to change descr_sm_factions at this stage too.

Changing culture to greek will alter appearance of buildings automatically, but some temple types are listed by faction so you'll need to find Selucid ones and alter ie. add numdia,
temple_of_healing_shrine requires factions { britons, egyptian, seleucid, romans_brutii, numidia, }
I think previously theie temples were just listed under carthaginian culture so when you change culture they will not have them anymore.