When calculating joined data, you cannot calculate the joined columns directly. However, you can directly calculate the columns of the origin table. To calculate the joined data, you must first add the joined tables or layers to ArcMap. You can then perform calculations on this data separately. These changes will be reflected in the joined columns.
Calculate Field (Data Management)
I know this is an older post, but I'm running into a similar problem...
I'm wanting to run the Calculate Field too on a combination of fields from both the table where the join table will be joined to and the join table itself. For example, I have a layer of Pipelines (the layer to which the join table will be joined) and a layer called Pipeline_Stats (the table to be joined to the input layer (Pipelines)). But when I run the Calculate Field tool, and choose the Pipelines layer as the Input Table, I only see fields originally contained in the Pipeline layer, and none of the fields from the joined Pipeline_Stats table. I could copy the data over I suppose, but I'm not sure which tool to use to accomplish this. Plus, that somewhat defeats the purpose of the Join, I guess.
This will be run inside a Model, so I can automate the process if it'll take multiple steps to get it accomplshed -- although the simpler/easier would be best...
Thoughts?