If you connect a load to a constant voltage source (eg a 5V power supply) then current will flow through the load. The load can be anything: a resistor, motor, circuit, etc.
The amount of current that flows through depends on the load and the supplied voltage. If I have a 100 ohm resistor as my load hooked up to a constant 5V power supply, I can calculate the amount of current drawn through the resistor using ohm's law (V=IR). For nonsimple loads it gets a little messier, but you can measure it instead of calculating it.
Your '5v 600mA' power supply is probably a constant voltage power supply. The amount of current it provides will change -- 600mA is likely just a maximum value.
If you have a 2.5V constant-voltage power supply, it will supply any amount of current up to its limit (where it will hopefully shut itself off rather than damage itself). If your load will only draw 500mA at 2.5V then all you need to do is get/make a constant 2.5V power supply that can supply at least 500mA without blowing up. On the other hand your load may want to draw more than 500mA when given 2.5V, so you would have to drop the voltage to compensate.