The phrase "are fully accustomed" is correct and usable in written English.
It can be used to indicate that someone has become familiar with or adapted to a particular situation or environment.
Example: "After living in the city for several years, they are fully accustomed to the fast-paced lifestyle."
Alternatives: "are completely used to" or "have fully adapted to".
They found differences between people who were born blind and those who lost their sight as adults, and between those who had only just begun to use a given device and those who were fully accustomed to it.
All the animals were fully accustomed to be handled.
This procedure was not well tolerated by all of the investigated animals, and implies than the investigated goat is fully accustomed to being handled, which is not always the case in caprine field practice.
Two of his corps commanders Richard S. Ewell and A.P. Hill had only recently been promoted and were not fully accustomed to Lee's style of command, in which he provided only general objectives and guidance to their former commander, Stonewall Jackson Jacksonn translated these into detailed, specific orders to his division commanders.
In the first place, of all the people on that address list, she is the only one who is both fully accustomed to cleaning up the owner's messes and also completely willing to wipe up the next one.
The newly wealthy blue half of Manchester is becoming fully accustomed to the disapproval of domestic rivals, but their opponents tonight, Bayern Munich, are past masters at carrying on, and succeeding, regardless of others' jealousy and hatred.
Albeit he is now fully accustomed to close scrutiny, the 24-year-old knows only too well what level of focus will be placed on his every move so close to home.
Gentle conversation between us, with no specific purpose or time limit, is something we are not yet fully accustomed to, something to be savored only occasionally, and with great care.
Elementary-aged children are still learning but are not yet fully accustomed to "church language," so their prayers are not encumbered by doubt over using the right church words.
Individual platforms such as the Sega Genesis and PlayStation 3 are seen as difficult to develop for compared to their competitors, and developers are not yet fully accustomed to new technologies such as multi-core processors and hyper-threading.
