You can and it’s easy enough as Jeremy says, with a caveat. You said PNG, so you should be safe. The issue could arise that their graphic doesn’t have an alpha channel (if I had a nickel for every time I got one like that…). If not, it will cover your Timeline and you won’t be able to use it. Either way, I bet you anything it matches what’s built-in to FCPX, because it’s an industry standard. I’ve never seen a situation where a TV station needed some odd “custom” zone settings.
When I ran TV broadcast, we sent a graphic out because we never knew the level or software of who we were contracting with. I’d ask what NLE they used, and had them explain action and title safe to me on the phone. If they knew, I was fine. If they didn’t or stumbled, we sent out a file with alpha channel, explained how to use it, and what it was for.
But you should be fine, it seems.