Not sure what you mean by amp that can you use as a sub.
If you you want to add a self powered sub to your 2.0 speakers how you do it will depend on the PC.
If you have a orange audio jack on your computer you can connect the subwoofer amp there. It has the center channel output too so you have to use the correct RCA output. Set your PC to 2.1.
If not then you can split the output you have the 2.0 speakers connected to and connect the subwoofer amp there. Set your PC to 2.0.