You can convert an existing SketchUp section plane to a Skalp Section by selecting the section plane and then access its context menu by right clicking. Select ‘Create Skalp from SectionPlane’
A dialog will ask you to enter a name for the new Skalp section. You can always change the name later on from the main Skalp dialog.
Optionally you can have Skalp Create a Scene and associate it with this Section. This association can also be managed from the main Skalp dialog.