Well, it isn't perfect, but...
(1) Plug in the external media.
(2) "START, Settings, Apps".
(3) Scroll to & click the app, & click the Move button that appears.
A new requestor appears with a second move button.
(4) Ensure the external media shows up in the requestor,
& click the second Move button.
This isn't perfect, because you will see lots of them don't have a move button: only Store apps will have it, I read. Also, many of those that do have a move button will have it shaded - it won't work. Anyway, here is how to have newly downloaded apps go there automatically...
(1) Plug in the external media.
(2) "START, Settings, System, Storage, Change where new content is saved".
Here are two articles I got that from...
https://www.ghacks.net/2015/05/04/how-to-move-windows-10-apps-to-another-drive-to-save-space/
How to move Windows 10 apps to another drive to save space
http://www.thewindowsclub.com/move-windows-10-apps-other-drive-change-install-location
Move Windows 10 Apps to another Drive