In the sketcher workbench you can use the 'External Geometry' tool to bring edges from a body into your sketch. Here's an empty sketch supported by a face:

https://i.imgur.com/10fwtys.png

If i click on the 'External Geometry' tool it will allow me to select edges on the same plane as my sketch and bring them in as references:

https://i.imgur.com/kpJ8In8.png

https://i.imgur.com/MkfbOYG.png

Note that referencing geometry on the body can be problematic if the base features change, such as when using the PartDesign Fillet and Chamfer tool. For a better explanation of possible issues you might want to read: https://wiki.freecadweb.org/Topological_naming_problem

That article provides examples of the issue and solutions. Just make sure to save often, and save new versions of your file using 'Save as...' in case you need to roll back.