Can I offer to my clients that I addon, to use the Cpanel for manage their own e-mail accounts and hosting ??

No there is impossible to assign a separate control panel for addon domains.
If you want to grant cpanel access to our customers you need to have a whm access.

So, If I buy the Private Label Resell Plan, I can add as many Hosted domains as I want, but how can I give each domain their own webmail interface (http://costumer.com/webmail).

Also, can I have the same e-mail account for many domains: webmaster@costumer1.com, webmaster@costumer2.com, because I'm actually with verio and I just can have 1 real e-mail address webmaster@costumer1.com, the other are webmaster2@costumer2.com

Thanks

Each domain name created under your reseller panel has its own account (so it has webmail).


To do this you need to create them at users' control panel.
Or, if it is your account, you can create e-mails for many domain names at your control panel (not reseller).