I don't see a manner in which to do this via the shape but since the shape is essentially a wrapper around the activity pxCreateCase which has a parameter SourcePageParamName for the datatransform I would suspect you could achive your goals by having a utility call a activity which sets parameters on the SourcePageParamName and then calls the pxCreateCase activity.
Unfortunately there is no way to pass params for two reasons here
Smart shapes are a wrapper over existing activities and this Data transform name is a parameter to pxCreateTopCase activity. And like regular utility calls there is no way to pass parameters to a parameter which is rule itself
Even if we set params in current param page it won't work since you have selected first option which creates a top case using pxCreateTopCase in which Step 5, we apply this data transform without passing current parameter page
However i believe instead of using parameters you can source those values from a temp top level page and populate that top level page properties with valid values from connectors.