place, trace, expand and stroke - stuck on stroke

I'm trying to write a script to iterate the files in a folder - I need to place, trace, expand and stroke each.

So far, I have the code below working nicely - I can place, trace and expand but can't work out how to add the stroke?

for ( i = 0; i < fileList.length; i++ ) { 

            lay = doc.layers.add();    

            lay.name = fileList[i].name.slice(0, -4);  

            thisPlace = doc.placedItems.add();                           

            thisPlace.file = allFiles[i];                         

            thisImage = thisPlace.trace();                                                                  

            thisImage.tracing.tracingOptions.loadFromPreset( 'myPreset' );  

            var thisTracing = thisImage.tracing.expandTracing();

}

I've edited the script for brevity's sake - everything else is working, I just can't get the stroke. I think the issue I'm having is knowing what object to apply the stroke to.

Any advice would be brilliant.