You cannot... rather you can, but since you don't have rights over the xshellz.com domain, a valid certificate could not be granted. This leaves two choices, add an exception and accept the self-signed certificate (it's not any less secure, there is just no chain of trust) or petition xshellz admins to create, issue and provide a cert for znc to use for shell.xshellz.com (the znc installer would need to be modified and I couldn't really say whether or not this whole process would be trivial or not without investigating further).
There is actually a fairly simple (and fully automated) solution that came to me while I was sleeping on the Metro ride home today (public transit FTW). I need to work with the xShellz staff, but for anyone who has wanted a valid/signed https connection for their znc (or whatever else uses a webfront) web admin page, this will allow for that.
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