No. A brep (what most users would call a polysurface) is a list of several different pieces of information, mostly surfaces and curves that describe the Boundry of the object (that's what the B stands for). Anyway, if you tap into this list, then you can simply duplicate one of the edges from that list. The big issues is actually how do you know which edge to specify, so you're going to have to figure out a way to find out which edge you want.
Dim myEdgeCrv as OnCurve = Nothing
myEdgeCrv = myBrep.m_E(0).DuplicateCurve
FWIW these questions should probably be going in the VB/C# corner.
I should add that a mesh is going to be a bit harder more because its probably going to be harder specifying the edge than with a brep. What you're really looking for, in regards to what you duplicate, is actually just the two edge vertice. Once you have that you can just make the line yourself.
Actually I haven't found a "duplicate naked edges" method in the SDK. Which means that finding naked edges is a real bitch. Why are you dealing with meshes within GH anyway? Yea they may stick their head in here or there, but if you're generating any sort of geometry, you might want to stick with NURBS as much as you can.