How do I add seats to this van?

Depending on how the van was optioned when it was purchased, it may be difficult or impossible to add seats, as the mounting brackets may not be there, and you may get into cutting, welding, or bolting brackets in for seats. The commercial registration piece may not be from the lack of seats. Here in PA vans and trucks can be registered by gross vehicle weight, which may be part of your problem. A higher GVW = higher registration. I've never heard of a van you couldn't register as non-commercial...?