That's possible, but not by just editing the stencilset.json. The type of the class-property will no longer be "string", but will be a complex type. You'll have to add some code the the oryx-javascript, to render a custom editor for that specific type of property, in your case, showing a popup-box that does all the calls to the service. This requires javascript-coding and some digging into the oryx-internals.