Yes, I now… but I’m new to Kanboard and asking for a ‘helping hand’ here (which could help to bring such an addition to the bulk modal as a plugin for example)
A ‘helping hand’ is a somewhat vague term. I’m willing to answer any concrete question, but I cannot (and don’t want) do the entire job.
Are you familiar with the PHP programming language at all?
I’d recommend taking a look at other plugins. How things are done, try to understand the architecture.
EDIT After inspecting the code for required changes: This will rarely be a beginner’s job, will get rather complex. Please be prepared and/or think about it.
Modify Kanboard files directly, as you did, create a pull request at the Kanboard repo and hope that @fred accepts it.
Implement if as a plugin. But then you have to override two templates, task_list/header and task_bulk_change_property/show.
In general, I don’t like overriding because it might interfere with other plugins in use and in case Kanboard changes the original template in the future, you’d have to apply the changes to your overrides. It’s up to you to decide.