can I import a pic and use it in a separate layer like a template so that I can sketch over it on a separate layer?
I'm not sure which version of Sketch you're using but here's how to add a photo layer in the latest version (3.5):
1. When you open a new Sketch artboard, tap the + above the Draw Layer opposite the toolset.
2. A menu will open with the choice of Drawing Layer and Image Layer. Tap Image Layer.
3. Then choose from where you'd like to import the image (On my iPad, Take a Photo, My CC Files, From Market, From [Adobe] Stock, Paste Image).
Let me know if you have trouble.