How to display custom properties of a document in the form of the workflow task-editor?

[yanli]

I want to display more custom document properties in the form of the workflow task-editor. I found the freemarker template file packageitems.ftl for this form. But how to extend this ftl to show a custom property?

[afaust]

That form control, its FTL and associated client-side JS sources, as well as the backend ReST API, were never intended to be customised in any meaningful way. Having had such a requirement myself years ago, I can tell you, it is not possible without extensive source/core patching, and you are better off considering to develop a custom form control of your own from scratch.