It looks like the current Jira IssueFieldUpdateStep operation doesn't support multiple values, which is very useful when using custom label fields in Jira which can be searchable.

I have created something that we use internally and I'm wondering how can this become part of the mainline release.

https://github.com/romanblachman/jira-plugin/commit/0cc4bbbed74335a2d194e54fede86224dd3b8aae