Thanks for your post! TestRail's milestones would currently only support the Name, Description, Start/End Dates, and Completed fields. It wouldn't be possible to add another milestone field that would be filterable/searchable via the built-in filter/search features. That said, you can use Markdown to format the milestone description, and you could then e.g. create a table to organize any data that you add to the description (e.g. to look like separate fields). From there you can just use TestRail's built-in search to search for any term, however keep in mind that this would search all entities and not just milestones. You can learn more about Markdown on our website here:
We're also happy to review support for additional entity fields as well. Hope this helps!