I can currently use the jira plugin to create a version of some software after a Jenkins build releasing that version. I can also use the "Move issues matching JQL to the specified version" step to move all issues mentioned in commits in that build into the version I just created.

This has options to either replace all versions with the new version, or just one version with the new version. Instead, I'd like to add the new version to the fixVersion field. This is to support e.g. bugfixes backported from another release. Another option I considered is a "replace only if no fixVersion already specified". Then we would at least not overwrite other valid fixVersion values, and we could look at ways of tracking the version dependencies separately.