it will require hand editing the webassist/kfm/configuration.php file.
the lines that sets the upload folder location is:
$kfm_userfiles_address = '../../images/';
$kfm_userfiles_output = 'images/';
this sets the upload location to the images folder of my site, it may be diferant for your site.
you can add a session to the path by changing that to:
$kfm_userfiles_address = '../../images/'.$_SESSION['userID'];
$kfm_userfiles_output = 'images/'.$_SESSION['userID'];
this is using a session named userID, you can change this to another session if you like.
bear in mind, that this is not a supported modification, you will need to be comfortable editing PHP to get this to work.