I found this function.
How can I now add a Stroke to my Drawable?
There is nothing like setStroke(...)
Try creating a paint variable with the your intended color/stroke width/ (also set style=stroke) and then use it in the onDraw method:
onDraw(Shape shape, Canvas canvas, Paint paint)
In this way, the stroke will be added when you draw. It will not be attached to your Drawable.