spazfirem is correct.
The process is also explained more directly here: https://unrealstudiohelp.epicgames.com/s/article/How-Can-I-Convert-an-Existing-Project-into-an-Unreal-Studio-Project
Note, that in the upcoming 4.20 release, this will no longer be an issue (if installed to the engine, Datasmith will be available in all project types)