Currently, there is no way to generate export files where content items have more than one categoryPath. The RecommendationClient allows sending only single categoryPath, ref: https://github.com/ezsystems/ezrecommendation-client/blob/4ea5f4485a0193d0b0891e4f274e0433980dc08f/src/lib/Service/ContentService.php#L180.
Specifying categoryPath as an array or comma-separated string does not make any difference.

On the API side, passed category paths are merged into one string.