How do you install WHM/Cpanel within a Virtuozzo VE?
The same way you would for any server, only one change for the network device in edit setup in WHM
Is there a template required?
No, one was (and may still be) available but templates are not a good idea for cpanel as it is updated daily, you would spend a lot of time making templates.
Any way to install it once (paying for 1 cpanel license only) on the server and share it for all VPS accounts?
No that isn't possible, you need a licence for each VPS. The guys at cpanel do a great deal on VPS licences for NOC partners, drop them an email. Trying to find a way around cpanels licencing may not be a good idea.