I feel like I don't quite have the full picture but it sounds like you want to map Account based on something that is not currently mapped in your data source. You can map this to any field such as an unused UD or attribute field and bring it in. Sometimes people might also concatenate 2 fields together in these situations.
I think as you have guessed, if you create a composite rule you will need to ensure all the 1:1 mappings that should run after the composite rule are also migrated to composite rules to ensure they run in the appropriate order. I hope this changes one day! You don't necessarily need to set up a wildcard though. If you catch your exceptions first, then anything that doesn't fall into those exceptions will naturally find your other account mappings without the need to explicitly set a wildcard
e.g.,
Rule 1: A:10000 Location 2000ABC
Rule 2: A:10000
Rule 2 is effectively a wildcard on location without needing to specify it.
Sorry it was hard to explain, but I think I will actually have to update my source setup for my account mapping to bring in that location field. I have a separate location mapping to the UD1, but now I need to bring that field into my mapping which means I need to update all my transformation rules. Thanks for taking the time to give me some input on it.
