Several methods are possible.

They don't all give the same result.
So you choose based on what you want to achieve.

First possibility I see
- Select your object
- Construct -> Offset -> Shell


Second
- Select your object
- Copy / Paste to have two surfaces
- Move one of your two object
- Select all external edges of your two objects
- Edit / Join it will join continuous edges and create 2 closed curves
- You can Construct / Loft between those 2 closed curves
- Next select your 3 objects top/bottom sufaces and the lofting
- Join then to have a solid

If you test those 2 methods, you could compare, see, and undersatnd the differences between those methods to choose the best for your case.

I think it exists many other methods but with those you have a first choice.

Have fun