[xwiki-users] change registration behavior

Hi! Is it possible to change default autocompletion of username in tregistration window? So when I'm enterning the first name and real name then the username autocompletes to FirstnameLastname (uppercased). I want to change it to first letter of firstnaeme and lastname without uppercase and set it by default so users could not change this pattern. When I clicked to edit the filds I've found tht this autocompletion is made by javascript script "prepareName(...)" Where is this script? can I change it when I've got hosted xwiki on myxwiki? If not is there any method to change this behavior as I want? Thank you! Wojtek