You can transform a closed area created by a line to a Polygon to Plate Section command. With this method, you can define a section in any form.
Location of Polygon to Plate Section Command
You can access it from the ribbon menu Modify tab under the Transform heading.
Create a closed area with the line.
Click the command polygon to plate section.
The Section Settings dialog will open. Set your section name and material.
Click the OK button to close the dialog.
The section you define will be listed under the Sections row in the Structure Tree view .
Structure tree before the sheet section is formed
Creating a closed area with a line
Selecting the area to be transformed into a plate section
Editing the settings
Creation of the plate section and adding it to the structure tree