you have to see first what ability mana stone has (or ring of regeneration).
then in ability editor modify field "data - mana regeneration per second" to be max number (to open field press and hold shift button, and then double-click the field) type 9999999999 and then add item into inventory of your hero... no mana loss... but, if you use that shift-clicking, like berzeker said, just hope that you WC3 won't crash. tru first "regular clicking" then if it's n ot enough try it with shift-clicking....