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don't quite understand... if it's already sheetmetal, unsuppress the flatpattern feature. If it isn't already sheetmetal, then you have to convert it into a sheetmetal part so you can then flatten it, bend it, etc. If it's a rolled sheetmetal part with two ends on the radii, select one of the end/surfaces and insert sheetmetal bend. This will convert it to sheetmetal and allow you to create flatpatterns, bends, etc.
First, I want to clarify that sheet metal parts need to have uniform thickness like a piece of sheet metal. Also, you say "round" but I'll assume that you mean a radius along a corner. If these are both true then you should be able to create a sheet metal part. Solidworks Help should walk you through examples if you aren't sure.
If your formed part does not have uniform thickness, you can't create a sheet metal part. If your part is round like a sphere or partial sphere, you won't be able to create a sheet metal part. Basically a sheet metal part must have some planar surfaces and radii that are individually 2D as opposed to compound curves (3D). Picture in your mind how you could bend a sheet of paper to form the part and all the features.
Solidworks Sheetmetal parts will not handle deep draws that form complex 3D surfaces.