What do I need?

If your 2-channel preamp does NOT have a HT bypass feature, I suspect your only option is to directly wire your 2-channel speakers like the rest of the surround speakers from your surround-preamp-processor.  

@kennyc That’s not true.  Any unused line-level input on the stereo pre can work very easily — I did it this way for years. 

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.