Import the images to external_image_url field, separated by | symbol. This field is a new custom postmeta field that you enter in whatever import tool you are using.

For alt text, I’m releasing an update shortly (should be this week) to provide this option. Currently, you cannot import the alt-texts – sorry about this! Currently, you can edit the featured image alt text, but there is no way to edit the gallery image alt texts or variation featured/gallery alt texts either.

This will all be fixed asap.