Secondly, would it unbalance my character too much with the rest of RBP? Finally, can you give me a hint on which esp(s) to change and what fields do I need to copy from and to...thanks in advance.
How to add: If it is only for this character, easiest way would be to use player.addspell ##001D76, where ## is the load order digit.
With tes4edit you would only need to edit the Core file. Open it, expand birthsigns, select the tower. There you'll find a category named "spells". Rightlick, choose "add". An empty line (NULL reference) will appear. In this rightclick again, choose modify, and copy in it this formid: 01001D76
Note however that the first two digits are the load order index in tes4edit. If you opened no other files, it's 01.
This will only work for new characters. For existing you still need to use the above addspell command.
It shouldn't be that unbalancing
