That's interesting. That opens the possibility for people with an expired exam to get a single engine ATP with a view to an add-on multi (or type rating) ATP added later.
They thought of that. 61.165 (c) says if you want to add a ME ATP to an ATP in a different category you have to complete the ATP training program. And 61.165(e) says if you want add a ME to a SE ATP you have to complete the program.