How do I create a numbered list that starts with '(aa)', '(ab)', etc.?

I want to create a numbered list in MS Word that starts with '(aa)', '(ab)', etc.? However, MS Word only offers '(a)', '(b)', etc. I understand that I might not be able to use MS Word's inbuilt 'Numbering' tool, but that doesn't bother me. My plan is to combine the '(aa)' style numbered list with the '(a)' style numbered list, so at least visually, both numbered lists need to line up exactly in terms of indents.