Add suitable reference geometry to parts, then 'show reference geometry' for the parts in the assembly workspace. Constrain using the reference geometry.
Through the Sketch menu you can make any figure type directly as reference (I don't use the ribbon so don't know about that)
or you can make a circle, right click on that circle and select "convert to reference figures".
Oh, you were referring to a sketch reference geometry. Yeah I can do that, but, as I said at the beginning, I cannot see sketches of these parts at the assembly level:
This is what I have in that gear part, when I have that sketch activated:
I won't ever get a sketch circle for my constraints, but I could just use a line or a plane tangent to that desired circle, and use it to align my gear with the rack.
Put a pitch line plane in the rack, turn on "show geometry" for the rack in the assembly, now offset the centre of the gear by half the pcd from the pitch plane of the rack.