Service Plan Option on Leads

When a new lead comes in we can send them a quote, but as far as I can see we can’t sell a service plan directly, we have to make them a customer first. I would love to be able to create the service plan and the agreement directly for a lead and upon acceptance they become a customer. The current work around isn’t too difficult, but if they are never serviced I’d like to keep them as a lead for long term nurture.