Currently, there isn't a way to import to a layer, or import multiple layers to an existing project.
There likely will in a future version - we want to improve the layer support.
However, there's a work-around if you seriously seriously must with the current version. It's a bit convoluted, and not immediately obvious, but here's what you can do.
Create your painting, and select the layer you want to add the image.
Load a tracing image - the image you want to add to the layer.
Click the 'Edit tracing image' button.
From the Tracing Image Settings pane, select the 'Scale' button.
Select the 'manual scale' option.
Now you can use the pan and scale buttons to the left of the tracing preview image to position the tracing image to the position you'd like the imported image to be.
Click the 'Done' button when you've positioned your tracing image.
In the preview pane, to the right of the 'edit tracng image' button, click the right arrow to bring up the layer menu options.
Select 'Convert tracing image to paint on this layer' option.
This will past the image into the layer at that point.
Then turn off the tracing paper overlay.
So to recap: Load and position the tracing paper to where you'd like the imported image, then convert to paint in that layer.
I know it's a lot of steps, but it's a work-around until we get some better layer options.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"