How do you import from library into existing layer?

Sometimes I want to do this instead of creating a new layer.

Is there a quick way to do this(other than importing copying from the new layer created and pasting into the target layer)?

You can drag your template/symbol onto the layer you want to import to on the right hand side of the timeline. You should see a yellow highlight that will show you the frames where your selection will drop. Note that if your template is in a hierarchy, the hierarchy of the destination must match the hierarchy of the source.