He can't "write" his own words. They wanted to make sure he wouldn't be able to override commands given by a real crew, so he has no way of entering--as in 'creating'--any "text" into the computer.
I haven't decided full scope of data ports access, but yes, they are also for communication with the "organic components" of the ship.
In a Cardassian library
or in a Cardassian gallery
"Reagan, it appears, is really only an ardent unionist if the unions in question are in Poland" - Stephen King, Skeleton Crew