Write to file from pgplsql function

Hello,

is there a way to write to a file in the filesystem from a pgplsql function?

The reason for this is the architecture of our user system: Each new database has a file in $PGDATA/users with a list
ofusers who are allowed to connect to this database. In pg_hba.conf is a link to this file. As some users don't have
filesystemprivileges, but are allowed to add users, they have to be able to call a function in postgres which adds that
userto the file. 

I appreciate your help, or a tip to improve the user system.

Sven Sporer