There are a number of possible ways to configure for 8 rooms with 2 receivers and multiple modulators.
Sorta all depends on your goals.
If you wanted to put the FTA receivers near the dish or maybe where the original cable came in, and take the outputs of the receivers (or modulators) and feed the house wiring with those, then sure, that'd probably work without much trouble.
The problem comes when you want to run the signal from the LNB to your FTA receivers through that existing wiring.
You're just better off -not-
That's the easy answer.
If that is your goal, then I'd get a 100 foot roll of cable, get my receivers and everything up 'n running on my spare cable laying down the hall, and then once it worked, try to use wall wiring.
But, only at the very end of the project.
As for installers, if we knew where you were, maybe someone would know a person or company in your area.
Of course, if you're patient, reasonably handy, and want to learn, you can find out how to do it all right here on the forum.