How to configure stencil of subgraph?

[vega at fahr-zur-hoelle.org]

Hi group,

I'm just playing around with the demo of Omnigraffle 5 and think the subgraph is one of its killer features. But: Does anybody know how to modify or define the stencil for the outer box of a subgraph? It seems only that boring plain box is available for it but not any custom stencil. 

Some hints would be very appreciated.

Regards,
Tom