As long as you are subscribed to the mod you want to use abilities from they will work. All you need is the abilities coded name which you can usually find in the XcomGame.int localization file (search for the name you know the ability by and it'll take you to a bit where it shows a display name for it, above that is the coded name in the [] brackets). For example in my class mod the "Seek and Destroy" perk is known as SeekandDestroyRS.
The only other thing you need to know is how to apply it, you can look up its coded name in the XComClassData.ini file in the config folder of the authors mod. Just search for the coded name and it will show you how it is applied. For example Seek and Destroy is like this:
(AbilityName="SeekandDestroyRS", ApplyToWeaponSlot=eInvSlot_PrimaryWeapon)
but something like Lightning Reflexes is like this:
(AbilityName="LightningReflexes", ApplyToWeaponSlot=eInvSlot_Unknown)

Publicado originalmente por Richard:

As long as you are subscribed to the mod you want to use abilities from they will work. All you need is the abilities coded name which you can usually find in the XcomGame.int localization file (search for the name you know the ability by and it'll take you to a bit where it shows a display name for it, above that is the coded name in the [] brackets). For example in my class mod the "Seek and Destroy" perk is known as SeekandDestroyRS.
The only other thing you need to know is how to apply it, you can look up its coded name in the XComClassData.ini file in the config folder of the authors mod. Just search for the coded name and it will show you how it is applied. For example Seek and Destroy is like this:
(AbilityName="SeekandDestroyRS", ApplyToWeaponSlot=eInvSlot_PrimaryWeapon)
but something like Lightning Reflexes is like this:
(AbilityName="LightningReflexes", ApplyToWeaponSlot=eInvSlot_Unknown)

Thank you)