Change simgle key with mod key behaviour in openbox and x11

I'm trying to change de behaviour of pressing key AltGR + z. Now, when I press this combination of keys, character « is written, but I would like to write < symbol.

I executed this command:

xmodmap -e 'keycode 52 = z Z z Z less guillemotleft' 

This command works fine, but changes aren't permanent. How could I make it permanent, or how could I change the behaviour of AltGR (mod1) + z?

Note: less: < guillemotleft: «

Thank you very much.