When importing modules into FortiSOAR using the configuration wizard and selecting 'Merge with Existing' as the bulk action, the behavior for field handling is as follows: any fields that already exist in the system are overwritten with the imported values. New fields from the imported module are added to the system, while fields that are not part of the imported module remain unaffected and are retained in the system. This option ensures that existing data structures are updated with new information without losing existing, but non-imported, fields.