Hello kapefyra,
Welcome to the Seven Forums.
I believe that XP has to be installed first. I don't think it can recognize the seven boot files and thus installs it's own and makes them default. Seven will see the XP boot files and add XP to the list.
You could try Easy BCD. Link here:
FREE Great Programs for Windows 7
There is an add button. It may do the trick for you.
Please keep us informed.
Gary