Unfortunately this is not possible at the moment. We are preparing great improvements in area of workflow for Sitefinity 4.0. The feature you requested, as well as custom workflow paths and custom activities will be provided. We will also try to include an easy graphical interface for defining workflow.
I am sorry that I was not able to help you more at this time. Let us know if there is anything else we can do for you.
the Telerik team
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