Gramps doesn’t have bulk modification as a built-in capability. (With the exception of Tagging.)
But, you can use the SuperTool add-on to add citations if you are comfortable with database operations.
Or you can design an “export sample & import replicant” workflow that involves:
- creating a sample of the citation attached to a person
- Export the single citation to a spreadsheet
- filtering a view of your target records
- exporting the filter view to a spreadsheet,
- Replicate the sample citation line, substituting in your targets
- MAKE A BACKUP OF YOUR TREE
- import replicated list
See the Import and try a few mass changes to the example.gramps tree. (It is always better to experimental butchery on a replaceable cadaver.)