If you want to know traits, look up the "...\Rome - Total War\EB\Data\export_descr_character_traits.txt". Then do a search for the trait you want. Like "Arrogans", you will find this section:
Code:
;------------------------------------------
;Trait: Superbus
; o Insolens: <Law> -1
; o Arrogans: <Law> -2
; o Superbus: <Law> -3

Trait Superbus
    Characters family
    ExcludeCultures barbarian, greek, eastern, egyptian, carthaginian, nomad
    NoGoingBackLevel 1
    AntiTraits Modestus

    Level Insolens
         Description Insolens_desc
         EffectsDescription Insolens_effects_desc
         Threshold 1

         Effect Law -1

    Level Arrogans
         Description Arrogans_desc
         EffectsDescription Arrogans_effects_desc
         Threshold 2

         Effect Law -2

    Level Superbus
         Description Superbus_desc
         EffectsDescription Superbus_effects_desc
         Threshold 3

         Effect Law -3

;------------------------------------------
As you will see, "Arrogans" is the second level of "Trait Superbus". So you use the code and put: (example)

give_trait "Caivsminvtivs Gallicvs" Superbus 2

Just search for any trait you want to give in that file and then use the trait group name and the level of that trait.