Eagle provides the "Merge Library" function, you can merge the libraries created at different time and scene.
- Open the main library (Refer to the library as A library)
- Click "Library" > "Merge Library"
- Select the library you want to merge(Refer to the library as B library)
- Finally, the contents of the A library are A+B library
Eagle won't automatically delete the library which was merged. You can decide whether you want to delete it after the merge is complete.