I'd ask them: www.whmcs.com has a "ask a question" link. And since it's a paid product, there should be some customer service.
I am using WHMCS and I am facing a lot of problems in dealing with clients with the tickets. Actually I want to manage the tickets systematically in order to reduce response time & increase client satisfaction. WHMCS has default ticket escalation system but it is not suitable for me. Is there any way to create own custom ticket escalation rules based on product?