I have feature layer which needs to be updated with new attribute information. The text field I need to populate already has an established coded domain. I need to update hundreds of records and don't want to manually update. If I field calculate the selected values aren't I adding a new value not associated with my domain? What is the correct workflow for this type of mass edit?
The field calculator will ignore the domain that you have set up for the field. It will be possible to calculate the coded values that are not present in the domain. Calculating those values won't add them to the domain.
If you want to add multiple values to the domain, you should use the GP tool Add Coded Value To Domain which adds a value to a domain's coded value list. Alternatively, use the Table To Domain tool which creates or updates a coded value domain with values from a table.
I suggest using Python for automating the workflow. You should have a valid domain with valid coded values before you will edit the values. I'd consider editing an attribute with values that are not present in the domain associated with a field a bad practice.