This is not a currently supported feature of the HTML Editor. If you wanted to add something like this you would need to implement it as a custom solution, but it is not something that we support.
One way you could do this is to have a directory specified in the users record, then when the user logs in you can store this directory in a session variable. You could then reference this session variable in the HTMLEditor > editor > plugins > kfm > configuration.php for the $kfm_userfiles_address variable.