Your proposed modifications sound good. If you want to import assets from the 3D shooter, you can open both projects and drag the assets you want from one into the other. If you want the enemies specifically to maintain their behavior you will need to import all of their dependencies (scripts they use, etc.) as well and make sure they are attached to the enemy prefab correctly. This can be a pretty painful process since some scripts from the 3D FPS are re-used in the 3D Platformer but others are not.
If you really want them imported into the project I would suggest creating a backup of your 3D platformer and then importing the 3D FPS asset package, while turning off anything that would be overwritten by the import, to get the assets and then dealing with any errors that pop up post importing one at a time from the top of the console log to the bottom.