FreeCAD can be extended with the following extensions:
If there are multiple bodies in a project and you want in a sketch to 'Create an edge linked to an external geometry', you will notice that it is not possible to create this edge from another body.
FreeCAD offers a solution for this, called a ShapeBinder. A ShapeBinder is an object which is referencing to a face from another body.
To use it do the following. Select the Part Design toolbar:
Create a body with a sketch with a rectangle with some holes in it. When finished, close the sketch and pad the body to 10mm.
Create another body and optionally give it a sensible name.
Create a new sketch in the body added in step 2 and notice that it is not possible to create an edge linked to the body from step 1.
If not done, close the sketch tool and make sure the body from step 2 is selected (Body shows as bold text)
Click on create a new ShapeBinder
Click on Add Geometry and select the face of the Body from which you need one or more edges. When done click on OK
Before selecting edges from the ShapeBinder object, make sure now to hide the body from step 1. This is very important!
The ShapeBinder object should be visible now. If not done in step 3, create a sketch in the body in which the ShapeBinder object is.
You now will be able to use the 'Create an edge linked to an external geometry' tool and click within the ShapeBinder object to select points.