workitem attribute customization: get userid from owner or creator
One answer
No. Not in the attribute customization JavaScript API in general. I must have answered this and related questions at least 50 times now.
Note that https://jazz.net/wiki/bin/view/Main/AttributeCustomization only really supports string types and enumerations.
Also see https://rsjazz.wordpress.com/2016/07/15/rtc-process-customization-what-you-can-and-cannot-do/ for a better idea what you can do.
In the Web UI if you know what you are doing and reverse engineer the UI you might be able to do more e.g. communicate back to the Server to get that information, but you shouldn't do that.