Insert charactrer in username

[Nicola Iotti]

Hi all,
        we use Freeradius as proxy.
Is it possible to insert a character inside the username every time in
the same position (i.e. before last six characters) using
preproxy_users configuration file ?.
For example : original username = Johnnyfavourite
username after preproxy section = Johnnyfav at ourite
So the server can use "ourite" as proxy realm..

And if it is possible, which is the right syntax of the instruction in
preproxy_users file?
If it is not possible, is there another way to do it?

Thank you