Alt Gr as a modifier key only in combination with alphabetic keys?

I use Alt Gr + numeric keys to write brackets etc, but I never use Alt Gr + alphabetic keys. Because of this, it would be very useful to me if I could use Alt Gr as a modifier key ONLY in combination with alphabetic keys. As far as I know, there is currently no option for this, but would this be possible by editing some xml/config file?