(Sorry for the double post, I can't edit)
Ok, after some research I found out the following:
Adding the mercenary_unit flag works, but all Hastati will be treated as mercs if you follow this way. And you will have to extract the Unit Cards and Unit Info Pics from the packs as well as editing the descr_model_battle.txt and add merc lines for the texture and sprite images (see this thread, too https://forums.totalwar.org/vb/showthread.php?t=38378)

However, I recommend to add a new unit called merc hastati. Here's a How-To:

First, change your export_descr_unit.txt in /data/ to add the new unit:

Code:
type             merc hastati
dictionary       merc_hastati      ; Hastati (optional)
category         infantry
class            light
voice_type       Light_1
soldier          merc_hastati, 40, 0, 1
attributes       sea_faring, hide_forest, can_sap, mercenary_unit
formation        1, 2, 2, 3, 4, square
stat_health      1, 0
stat_pri         11, 2, pilum, 35, 2, thrown, blade, piercing, spear, 25 ,1
stat_pri_attr    prec, thrown ap
stat_sec         7, 2, no, 0, 0, melee, simple, piercing, sword, 25 ,1
stat_sec_attr    no
stat_pri_armour  5, 4, 5, metal
stat_sec_armour  0, 1, flesh
stat_heat        3
stat_ground      2, 0, 0, 0
stat_mental      6, normal, trained
stat_charge_dist 30
stat_fire_delay  0
stat_food        60, 300
stat_cost        1, 440, 170, 50, 70, 440
ownership        slave
Now we shortly dip into export_descr_unit_enum.txt (This doesn't seem to be necessary, but we'd better be sure before we leave it out) and add:
Code:
merc_hastati
merc_hastati_descr
merc_hastati_descr_short
Then you'll have to change descr_model_battle.txt in the same directory by adding these lines:
Code:
type				merc_hastati
skeleton			fs_javelinman, fs_swordsman
indiv_range			40
texture				merc, data/models_unit/textures/unit_roman_hastati_brutii.tga
model_flexi_m			data/models_unit/unit_roman_hastati_high.cas, 15
model_flexi_m			data/models_unit/unit_roman_hastati_med.cas, 30
model_flexi_m			data/models_unit/unit_roman_hastati_low.cas, 40
model_flexi			data/models_unit/unit_roman_hastati_lowest.cas, max
model_sprite		merc, 60.0, data/sprites/romans_brutii_roman_hastati_sprite.spr
model_tri			400, 0.5f, 0.5f, 0.5f
Note: You can use the sprites and textures you'd like to (or even add some new), but I simply took those of the Brutii because they are green ^^

Next up we will change the descriptions of our new Mercenary Hastati:
Go to /data/text/ and fire up export_units.txt and add:
Code:
{merc_hastati} Mercenary Hastati
{merc_hastati_descr} (Any description you wish, you can also copy that of the roman_hastati)
{merc_hastati_descr_short} (See above)
The next step is getting the info image and unit card right:
Extract the ui packs (or all packs) using Vercingetorix' Pak Tool
Now you have to create a few new folders in your /data/ directory which are:
/ui/
/ui/unit_info/
/ui/unit_info/merc/
/ui/units/
/ui/units/mercs

Next go to /data/packs/DATA/UI/UNIT_INFO/ROMANS_BRUTII/ and copy ROMAN_HASTATI_INFO.TGA to /data/ui/unit_info/merc/ and rename it to merc_hastati_info.tga
Go to /data/packs/DATA/UI/UNITS/ROMANS_BRUTII/ to copy #ROMAN_HASTATI.TGA to /data/ui/units/mercs/ and rename to #merc_hastati.tga

Last but not least you'll have to change your campaign descr_mercenaries.txt in order to be able to recruit the new merc unit you made.
So go into your favourite campaign directory and change the pool you want to have them in (I decided for Northern Italy) or add a new pool with your preferred provinces:
Code:
pool Northern_Italy
	regions Etruria Umbria Latium				
	unit merc samnite,			exp 0 cost 750 replenish 0.03 - 0.05 max 1 initial 0
	unit merc hastati,			exp 1 cost 750 replenish 0.05 - 0.1 max 1 initial 1
Note: You can use whichever stats you want, replenish is the amount of hastati-units which are created there each year (when it reaches 1 you will be able to buy new ones)
Now start your campaign, take the Julii and enjoy your newly created Hastati Mercs.

One more thing to say: Be careful when changing the files, an omitted comma or a space too much can literally screw you...

I hope this one helped you and I want to thank ikotw for his tips on how to change the unit card and info image in the above-mentioned thread.

Greets
alpaca