[Haskell-cafe] Parallel numeric class hierarchy

Let's say for arguments sake I want to make a new finer grained numeric class hierarchy. But, I also want to have the following occur: (1) Existing modules which have defined data types which have Num instances can be used with functions in my new hierarchy (2) New data types defined in the new hierarchy automatically get Num instances if appropriate so they can be used in existing modules. I know this is somewhat of a vague question but roughly how would I go about this?