You can't reasonably accomplish what I think you're trying to get at (since you seem hesitant to give us specifics on a receiver count).
You would need to physically remove EVERY DIRECTV receiver from the network when using FTA and vice versa (perhaps via appropriately rated A-B switches at every location -- even the locations that don't have FTA receivers) as well as A-B switching in the desired antenna when shifting between the two. If you missed just one of the receivers, the system wouldn't work.
Some switching may or may not be avoided through use of DIRECTV Genie Mini clients off of a Genie and diplexers (not recommended) or going with wireless minis but that's going to be costly if you aren't already set up that way.
If your existing cable is stapled in place, I'd give up now and add another set of cables to the FTA rooms otherwise you might use the existing cable to pull through some twin cable.
This assumes that you know what you're doing with respect to using multiple FTA receivers on a single cable to the FTA dish as sharing can have significant limitations (not just IF signal loss from splitting and/or diplexing the line).