Numbering like 1a, 1b, 2, 3

[XYZ]

I'm trying to do the following list in my document but I really don't
know how.

1a) Lorem (level 1)
1) Ipsum (level 2)
2) Dolor (level 2)
1b) Sit (level 1)
2) Amet (level 1)
3) Consectetur (level 1)

1a) and 1b) is the problem here as they have to be on the same outline
level as 2 and 3. Is there any way to accomplish that? I'm using Word
2010 Beta but I believe it doesn't really matter here.

Thanks.