What do you mean by "supply more current to motors"?
The motors will attempt to draw the current that they are designed for, for the voltage you supply. This is basic Ohm's law.
If the voltage source is not strong enough (high impedance) then the voltage will drop, and you won't get the current delivered that the motor wants to draw.
If the voltage source is strong (low impedance) then the voltage won't drop (much) and you'll get the current draw that the motors want.
So, what is the problem you want to solve? If you measure the voltage across the motors, is it lower than what the power source is rated for? If so, it's likely that your power source is too weak, and you need a lower impedance power source.
In general, LiPo batteries have very low impedance; NiHM batteries are OK; "regular" batteries are terrible. Plug-in power adapters are good up to their rated max output current, and then hit a brick wall with built-in protection.