Howdy! Welcome to our community of more than 130.000 members devoted to web hosting. This is a great place to get special offers from web hosts and post your own requests or ads. To start posting sign up here. Cheers!
/Peo, FreeWebSpace.net
I don't use HostBill but by taking a few minutes on their website, I can answer your questions.
1. Yes HostBill can import cPanel accounts. (Link)
2. I'm unable to find anything about custom payment gateways, although they support 60+ gateways. If you contact them, I'm sure they could provide assistance with creating your own.