Sure.
Do you already have clients using SSL? If so, they've already got Secure webmail, both Squirrelmail and UebiMiau.
They just need to use:
https://www.<theirdomain.com>/squirrelmail
or
https://www.<theirdomain.com>/webmail
For your users who don't have SSL, the best way to do this depends on whether or not you've already got a domain name you use for shared SSL.
If you already have a Secure Site Certificate for shared access, for example, example.com, then you can just tell your clients to log in as follows:
https://www.example.com/squirrelmail
or
https://www.example.com/webmail
If you don't have a Certificate and account for share access then you should log into the control panel as admin, and click to reach your admin user control panel.
From there, if you don't have a site, set one up. If you don't have a suitable domain for a site that you and your resellers will be comfortable using, get one.
Then buy a Secure Site Certificate for that domain, and install it at your admin user level control panel.
Then, presuming the site you just set up and bought the certificate for was example.net, give your users this information:
https://www.example.net/squirrelmail
or
https://www.example.net/uebimiau
For your users without SSL, you'll have to tell them the domain to use for secure access instead of letting them use their own domain name, or else they'll get a name mismatch error.
And don't forget to make the necessary changes to the welcome email(s). And don't forget that final
s in https.
Jeff