Can you put smart folders in folders?

The code for the Smart Folder is stored in ~/Library/Saved Searches.

You can double-click on a Saved Search file to open the search, so you could save those in a folder, but it won't act like a folder short of double-clicking to open it.

A "Smart Folder" is not a folder. It is a set of search criteria such as "name contains 'frogs'" and "date created is less than 5 days."

That is what is saved to a "Smart Folder" in the file in "Saved Searches" (or wherever you choose to store it).

There is nothing stored within a "Smart Folder" as it is not a folder at all. The items that the search criteria matches are "shown" to you in what appears to be a folder window. Those items can be stored anywhere within the scope of the Smart Folder's search criteria.

So, no, you can't store a "Smart Folder" inside another folder. You can store the search criteria file inside another folder.