Insert charactrer in username

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@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