ftp prefix doesn't allow the dot character

I went to:

control panel > Interface > Main Config > FTP user prefix​

and changed it to:

[CLIENTNAME].​

And got the error message:

Char not allowed in ftp user prefix.​

But, I am able to create ftp users in the Sites tab such as:

.mysite.com​

and get a FTP username such as myclient.mysite.com. In general, I like to make the FTP usernames for a given website to be the client name followed by a dot followed by the website name.
How can I modify the ISPConfig code to allow a dot character following the [CLIENTNAME] in the prefix? Can this restriction be lifted in future versions of ISPConfig?