Each category as you've realized has its own collection of parameters based on how they are used within Revit. Since the data you're describing is based on fixed DWG entities that are not going to vary (parametrically) you can create your own parameters to store the values you want to include in a schedule. You have a couple options:
Project Parameters: These can be created in the project and can be assigned to one or more categories. Then each family (of the chosen category(ies)) in the project will have this parameter available to display in a schedule. This allows you to store fixed data in each relevant family, but without having to add it to each family itself outside of the project. If you schedule these families you will likely need to filter out families that you don't want to include with your DWG based elements. Filtering is based on parameters too so it could be as simple as filtering based on the type name you use, making sure to a unique name or prefix that makes all your DWG based families unique for the schedule.
Family Parameters created from Shared Parameters: These parameters are defined in each family separately but from a shared parameter file (think dictionary, where parameter definitions are stored). You create a new parameter in each family and browse to a Shared Parameter file (dictionary) and select a parameter (the defined "word"). Shared Parameters are able to be tagged as well as schedule in the project environment, which makes them much more valuable/flexible...though slightly more work.
You can also create a project parameter based on a shared parameter definition.
Steve Stafford
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