Add callback to existing migrate field mapping
With some debugging and looking at code, it turned out to be pretty easy. All of the mappings are MigrateFieldMapping objects in $this->codedFieldMappings, so it's just a matter of getting a reference to that object and calling the callbacks() method. For example, to add a callback to the body field, you would use:

$this-&gt;codedFieldMappings['body']-&gt;callbacks('my_callback');