Most DirecTV dishes are too small, and generally have the wrong LNB.
Also, an FTA receiver will get nothing when aimed at the DirecTV satellites (through their dish).
If you are anxious to do some reading, below are some links to relevant threads.
But most of those conversions are not really recommended, and probably not for the beginner.
Too small indeed! 80Cm or bigger is recommended for the FTA hobby! If you’re talking about those pizza size 18” dishes of the past, forget it only to point at NASA! Bigger is always better here! I have a Direct international dish and that is about the closest you can get with a provider dish for some FTA but I see it is still undersized for some KU birds for my tastes! Anole has good reading posted there. DirecTV is has very different DBS broadcasting style than conventional KU so there is nothing you can get DirecTV with a DVB KU FTA receiver!
Do some reading around here first before you buy or try! We have a search feature at the top of the forums, punch in your question and I am sure that it will return an answer for you that is satisfactory! If it can be done, somebody here has done it for sure!