Adding Beaters
Sadly, I doubt that any kind of glue will withstand the stresses involved, Parwaz. A 'professional' modification would involve machining a groove (or even a 'T' slot) into the brushbar, and fabricating a plastic strip to fit it. You may get away with drilling your brushbar and using (lots of) self-tapping screws to secure your beater(s). You will have to be careful not to upset the balance of the brushbar. The advantage of this approach is that if it doesn't work, you can just remove the screws and beaters and carry on using your brushbar. ;-)
All best
Dave T