Hello Aditya,
You can use SolidExt1 (Beta) tool to make changes to any solid bodies. Or you can add a new sketch to imported geometry and make necessary changes in Design Modeler.
There's an inbuilt parameters tool available in DesignModeler to parameterize your dimensions. However, this is a much simpler process when you use Discovery or SpaceClaim. You can simply use pull tool and make the change parameterized using ruler option, that pops up when pull tool is active. Refer to the screenshot below from SpaceClaim(Ruler tool is marked in red).
Regards,
Nanda.
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