How can I make solid geometry in SheetMetal by Creo 2.0?

Nice find on the hidden feature! Unfortunately you never know when support for such options will be unsupported.

The only way I have been able to get solids into sheetmetal is to Switch to Part Solid in the Model>Operations tab. That is a drastic step, however, since it is not easily reversed.

The best solution (IMHO) is to create a next level assembly to add the solid to the sheetmetal. this is how I create inseparable assemblies where press-fit fasteners are installed. I tend to do some welding requirements at next level assemblies as well. This gives me good control over what I want on the fabrication drawing.

If you plan on thinning this to the sheet metal thickness later, however, you could look into the form features.

I'll have to look further into the hidden feature you found.

My question is how are you planning on manufacturing this in the end? I ask because, historically, Proe / Creo has ben centered around building how you would manufacture it. That's why you cannot make 'solids' in sheetmetal. In real life, sheetmetal is a constant thickness, period. If you want to add a mass to it like you've shown above, you make another part and join it to the sheetmetal by fasteners, welding, adhesives or whatever.

So the best way to do what you're showing would be to make two parts and assemble them together.

I tried the config option as well but didn't see what difference it made either. being a hidden option, it's going to be very hard to find any documentation on it.