If the requirement is to show it on the same column, this PROBABLY won't be possible OOTB . On different columns, which I am sure you've tried, will be possible.
For the same column, below are the 2 choices I can think of:
Function alias - although the logic to get this done is something that would take a few head scratches.
Customised section (not recommended) - Create a section with a table that calls this report. This gives you a lot of flexibility for what has to be displayed in the column. I imagine you can manipulate the data to be displayed on the "defer load" activity of the layout. Call this section in the report definition (display section for rows option).
For the second option, you will have to create your own export to excel button within the section you will call in the RD, quite a few OOTB activities for this (the default export to excel of RD has never worked in such cases in my experience).
Do post the solution here if you've figured it out.
Function alias is the way to go here. Let the database do the work. As far as I know, there is no OOTB function to do this, so you will have to write a custom one.
I would actually break this down into 2 FAs:
getOperatorName(operatorID) - Returns null if no such operatorID.
getAssignedToName(assignedTo) - Calls getOperatorName(assignedTo) - function aliases can be nested. If that returns null, return the assignedTo parameter value, as this will be the workbasket name. If it does not return null, return that value, since it will be the operator name.
These FAs are usually just structured as SQL CASE statements.