You can already create such a list. Just do it like this:
Click the numbered list button
Type test<enter> 3 times, resulting in your first list
Place your cursor on the end of the first item of the list
Press enter
Click on the Indent control in the CKEditor menu
Type Sub1<enter>
Type Sub2<enter>
Press enter
Click on the Indent button again
Type Sub-Sub1<enter>
Type Sub-Sub2<enter>
Type Sub-Sub3<enter>
You have the exact list of your example!