You would probably be best served with a FTA receiver that had an operation ANT/SAT option and putting up an entire FTA system. Then run the output from the
DTV receiver to the input for Antenna on the FTA receiver. To watch FTA you set to SAT, to watch DTV set to ANT. You really should be using a dish of 80cm or larger for FTA. The other issue you will have is lnbf. You currently have a circular, linear is required for FTA.
So if you have a small dish (normal DTV) you will need to purchase a dish, lnbf as described in a previous post, motor & receiver. With that you have already purchased the entire system. The only real difference with what I post is a cost savings for the lnbf. A linear is far less expensive than a combo unit. You would also be able to watch DTV at the same time as potentially viewing a different channel on a different sat with the FTA system. Just depends on how you wire it.
If you use 1 dish when you move off of a DTV sat you will not be able to view DTV channels until you move back. If you are running multiple DTV receivers it could prove a problem with others in the house.