How to import a plate from DXF
Create a new Stiffening plate and set the option Polygon in property Shape. Now you can choose command Dxf to open your *.dxf drawing.
Then click on the edge of the shape to pick up the line to the selection. You can manually pick up the other edge lines with pressed Ctrl button or click on the command Consecutive in the top ribbon. You have to pick up the closed curve, then click on command Outline.
The shape of imported plate is drawn in the Details window.
In the case of opening, pick up the closed curve and choose command Opening.
To finish the selection, click the button OK.
See the workflow in the video.