What do I need?

What is your amp and preamp?  Does your preamp have a home theater pass-through?  A pass-through will take the front two channels from a surround processor and deliver it to your two channel amp without affecting the signal (the volume is controlled by the surround processor/receiver).  I do this by using a Onkyo receiver as a surround processor (my preamp has a home theater input).  The Onkyo also has amplifiers for the center and rears, but I'm sending those channels to separate amps.

@tochsii You can power a home theater with 2 channels just fine. You will not get any surround functions, but you can't miss what you never had. 

If you’re gonna now have additional speakers for HT the cheapest/easiest thing would be to get an A/V receiver with front L/R pre outs that you can feed to your stereo preamp and power the other speakers.  If your stereo pre doesn’t have a HT bypass this can still work very easily using any unused line level input on the preamp, which I can elaborate on if needed.  

Get a good sub and run it either off pre outs or speaker level, whichever you system dictates and be done. 
 

I have been running 2.1 so to speak for 20 years in various systems and it has always sounded great. 

Look into buying a used Classe SSP-800 AV processor. It has an LFE out for a sub and you're good to go. Does your preamp have Home Cinema Pass through?