The MTH battery will charge just fine on your "9V" charger. I wouldtry it and see --- but do not charge for more than 1 hour. The 8.4V is the nominal output under load-- 1.2V per cell, 7 cells. A NiMH battery will charge to 1.4V per cell if the charging voltage is high enough. This is not a problem, as the voltage per cell will drop quickly to 1.2V, then maintain that voltage till it drains.
hope this helps--