If you import products using WebsitePanel, they will be imported using the Universal Module, and will continue to be billed normally. Blesta just wouldn't be able to auto provision new orders, or provide any management features.
You should do a test import. We're continually improving/adjusting/refining the importer, it works great for a lot of people, but some people have issues, it all depends on what your data looks like and how you are using it. Updating it to import KB articles probably wouldn't be terribly difficult, so if that's the only hang up then we're happy to work with you to add that functionality and you can help us test it.
You found the thread on WebsitePanel, I wonder if ModulesBakery still has plans for it. WorldPay is part of CORE-149 (I just made it public), not a ton of requests for it, but we might be able to help get this one built.