I'm using it to extrude my shape (or a cut that doesn't matter). I'm making a nest for a part to rest in.
What I did was make some points on copied spline then sketched a spline over the copied and mirrored spline. It worked but Im not sure thats the best way. Maybe bring the imported part into my part and combining them would be better then copy entities?
You could try a Fit Spline. I don't know what the down side would be, as I've never used them on other splines, just on lines and arcs.
I will try and find the post by Andy, one of the SW employees. There is a way to combine two splines into one.
Click on both splines, and chose the constraint "curvature" to give them equal tangency and curvature. Then you can click on the point between them and press the "delete" key. This will turn it into one line spline.
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