Looks @ Daniel..
#1 Just click the Add to.. istead of Create New..
Also you have to have your previous shape layer selected to add to it.
#2 Simply make sure the template is on top of the shape layer or layers.
Here's a self-explanatory screeny..
2 White Shapes in 1 Shape layer.
Note: It's a screen shot inside a screen shot.
Layer 2 is a screen of were the create new shape layer button is.