Switching heat bed

Hi,
I have old rapman 3.1 for which I build heated bed. The firmware allow to connect the heat bed to I/O of extruder 3. I have beed alble to connect the thermistor and the temperature measuring is working but problem is power. The one-board electro,ics has 12v output for extruder heater which of course is not enought for heatbed. I need to connect external 12v power supply. Is it doable to connect switching transistor on output of another (smaller) switching transistor to somehow proportionaly switch the load?

Thanks.