Adding Home Theater To 2 Channel System

This is very easy to do. I have been doing 6.1 (7.1 with a phantom center). 
Millercarbon is right. You do not need a center and they all suck (for the most part) anyway. Surrounds and subs are needed for sure. 
General:

You just need a receiver which can be the cheapest one you can find with preouts for the left and right front and you send the preouts to you two channel system. A home theater bypass is a nice feature but not needed. 
If you don’t have a bypass you can just use an open analog input and set the volume to the same spot every time on the two channel preamp.  When you watch a movie you control the volume through the receiver. At that point you just hook everything up as normal. 
Rears:

I personally would not spend a lot on rear speakers. I use 4 Klipsch RP8000f in piano black for my rears and just change put my fronts to what ever as normal for my two channel hobby. Another cheap option if you don’t mind the look are JBl 590 or 580s for rear speakers. 2 of them would work well and are super cheap with a horn tweeter. JBL blows them out for 1/2 price twice a year or more.  I have seen 590s go for $900 a pair and honestly they are pretty good speakers. 
Even though the klipsch are 100% different from my Thiels up front I have zero issues with a miss matched sound front to back. It just does not matter. 
Sub:

Subs are critical to home theater. I have two JL audio E112 subs and use them for both two channel and home theater at the same time. They have two inputs each and those inputs are summed. So the two channel preamp is run to each sub and the the LFE low frequency effect signal from the receiver is run to the other open input on the sub. 
This allows the subs to work with the high pass crossover for two channel and I can still adjust the gain of the sub through the receiver for movie sound tracks. 
Screen:

If you put it on the front wall and leave your speakers where they are you will never hear it. Honestly they are thin and soft so they are hard to hear between your speakers imo anyway. Screen innovations makes a nice hanging system you can hand from the ceiling too. I hang mine about 3’ off the rear wall and have my speakers 6’ out into room as a compromise for both screen size and viewing angles for the other seats…

but go big!  At 12’ Away I would want at least a 130” screen. 

That system will probably sound better in Stereo than trying g to hodge podge a HT around it....   I used to have a preamp with a HT pass through and ditched the 5.1 for 2 .1 and never looked back

home theater gear will have zero effect your two channel. They can run 100% independent of each other.
A 4.1 setup is so much better for movies than 2 channel. Much more involving. Speaker placement is key just like two channel it makes the biggest difference.

millercarbon is right, a good system does not need a center channel. Doesn't need rear channels either. It will need killer subwoofers.
I also have a 2 channel theater. Check out my system page and you can see it. Just hang a screen between your speakers and you are good to go. Not quite. It took me several weeks to get all of the rattles and buzzing out of the screen and it is a Stewart!

I'd replicate the advice/sentiment shared already and forego a center channel. If ever that route would have to be taken the only true solution would be 3 equal front speakers, period, but then you'd need a perforated screen + projector for that to make any sense. 

What I would do instead, and that wouldn't only serve Home Theater duties, is investing in a massively capable sub set-up, which is to say loads of displacement, headroom and no less than 20-25Hz extension (some may say honest 15Hz is required for HT to cover most, as in 99% of the infra-sonics found on the many Blu-ray/4K UHD titles, but a lower tune impacts the overall sensation of the bass - i.e. central to upper bass, and hereby integration in particular - so it's a compromise). This is MANDATORY for movies, but properly integrated (which goes without saying) will serve music reproduction as well. 

As for the particular sub solution (and ULF capabilities) to choose here, I'd forget about über-expensive subs and instead amp up their number for a DBA sub set-up, or go with 2 very large ones. If acoustic live performances are very much your cup of tea and form a reference for your home audio reproduction, I'd place the subs (say, 2 or 4 of them) symmetrical to the mains and coupled in stereo. If not a mono-coupled DBA placed asymmetrically may float your boat. Either way, never skimp on displacement and headroom, and get sufficient extension.