In general, you should find materials in bella to be simpler to create and use. The core material types are simpler and more elegant to begin with; however, they are abstract, so we are also adding various “semantic” material types, which encapsulate high-level concepts (plastic, glass, etc) with the minimum number of parameters exposed to provide necessary control.
This will help a great deal with conversion, since it becomes possible for you to express that a given collection of third-party materials should be interpreted as a certain type — that is to say, where it is remarkably difficult to take an arbitrary collection of parameters from a given material, and analyze them to find whether they should represent a plastic/glass/etc, it becomes a much more dependable process, when you are able to say “I consider these to be plastic, so see what you can do.”
Regarding assignment, though it is a given that a plugin must integrate with the host-application’s approach for managing material assignment, I think that your desired workflow can easily be accommodated using a spreadsheet approach. Just thinking out loud, but this could be something quite interesting to support in the bella scene itself (i.e. a node containing associations between materials, and either other materials, or external files), since it would allow swapping material setups without reassigning anything.