Thank you for the information. I did a few tests and post the result here in case anyone else wants to mod alien psi abilities:
1. You can define psi-amp items and configure them as you want.
2. You can overwrite the default psi weapon when you give the unit a "meleeWeapon:" with battletype:9 (the item does not appear in the inventory)
However, you cannot modifiy the stats for mind-control and panic independently, because the specific action cannot be chosen. I do not know how the AI choses the attack type. (If the player controls this unit, the attack is initiated by the psi icon in the upper right corner, without the possibility to choose the attack type).
3. You can give the unit a "builtInWeapons", which is basically a fixed weapon psi-amp (uses up an inventory slot). This can be configured freely.
However, this does not overwrite the default psi weapon, which means the unit will have two different psi weapons. I do not know how the AI chooses which psi weapon to use then.
EDIT: For testing purposes, it would be good if the player could choose the attack type (mind control or panic) when using the psi icon (top right corner) of psi capable unit.