Ist there any cheat to give Kaji better abilities in open lock/disable device?
Last playthrough Kaji was unable to open some doors in the Death God's Vault, I would like him to open them next time.

I don't think there is a cheat which allows to raise thief abilities of a familiar. You can however edit them in NWN2 Toolset, if you know how. You can also increase Kaji's abilities by using Heroism or Cat's Grace spells on him (or the Greater versions of these).

Does someone know if it is possible to open the doors with the knock spell?