To be clear I have no experience with the SheetMetal workbench.
When I look at the bottom of its wiki page, I see a link to Macro Sheet Metal Unfolder which looks tantalizingly relevant.
The macro can unfold simple sheet-metal-parts. The parts must have everywhere the same material thickness as typical for sheet-metal-parts.
Select a flat face from your part in the 3D-view. Start the macro.
Doesn’t look like it has gotten a lot of attention lately.
As I understand it, if you design the part in FreeCAD SheetMetal Workbench, unfold capability is included.
If you import it from somewhere else, I have no idea whether it works. The sheet metal unfolder macro hasn’t been touched and so would probably need to be touched up to work with current FreeCAD (just a guess).
It might be possible to use an imported part like that for external references to first make a native FreeCAD copy with the SheetMetal workbench, and then unfold. Or unfold might even work on the imported part!
OK, the suspense was killing me.
I just opened FreeCAD, created a sketch, put some folds in it in the SheetMetal workbench, exported as a STEP file, then in a new document imported that STEP file, clicked on a surface, and clicked Unfold. I chose ANSI K-factor and instantly I had an unfolded part.
@Kurt_Meister it looks like Unfold in the SheetMetal workbench will do exactly what you want, regardless of whether you design the part in FreeCAD or import a STEP file.