I came across this problem last year when planning a west to east coast road trip, the hire company wanted to add what I considered a huge charge to return the car to the original start point
We ended up doing two road trips, west coast into the centre and back out again and this year east coast into centre and back out again, which for us we believe ended up been far better than doing it all in the one trip
IIRC there are some hire companies that do allow this but normally it has to be within the state of hire, there are also some flydrive packages that allow you to do this as well which if suited you could be tailored for you.
Where are you intending to fly into and out of ??