Adding Home Theater To 2 Channel System

Don't do it. Doesn't your stereo image well? So what do you need a center channel for? Don't you have a wide expansive sound stage? The worst thing you can do to a fine system like yours is add a surround processor, of any kind. Any price. Does not matter. Instant 20% drop in system sound quality, even when listening to music in stereo. The movie sound field will be worse than in stereo. All you gain is some cheesy surround effects, that never will be anything like the holographic seamless organic world you can get from stereo.  

Not for lack of trying. I tried nearly two years. Two wasted years. I say wasted, because instead of anyone learning from this they all go and repeat the same mistakes. Build a whole new room if you must. Put a screen between your speakers. Anything but add HT dreck to an otherwise perfectly good stereo. https://systems.audiogon.com/systems/8367

It can be done BUT you need a preamp with bypass that puts the processor out of the loop for 2 channel, a good pre pro will have room correction dirac or some such that will MOSTLY take care of sound mismatch between speakers

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