multisite with email and ftp access


I would like to switch to multisite/prosite with automated provisioning (whmcs) and pretty plugins (resell outside prosite packages)..

However, how would I be able to have clients manage their own email and ftp?

Can I convert existing customers with their own cpanel/wordpress/ftp/email to my multisite isntall?

Any other tips regarding multisite and above setup that i should keep in mind?