Change Apple Music location to an external hard drive with an existing music folder

I would like to change my Apple Music folder location to an external hard drive.

The external hard drive music folder has 500GB of music and there is 40 gb on my iMac hd.

How do I create one Apple Music folder on my external hard drive that incorporates both the existing Apple music folder and the music folder on my hard drive?

Is it as easy as reassigning the external drive as the new music folder in preferences and copying the existing music on the internal drive to that folder or are there other considerations?

Many thanks for any input.