How to add 3rd person starter content to my first person project?

For transferring assets, you can simply migrate them using built-in ways. Select the assets you want to migrate either as a whole or individually, right click, choose “migrate actions” options from the menu that pops up and click on the “migrate” button.

For replacing the animations, you could retarget them but it wouldn’t be a problem since you mentioned that they use the same skeletal mesh.