Numbering 1a 1b 2 3

[XYZ]

I'm trying to do the following list in my document but I 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)

The real problem is 1a) and 1b) as these are to be on the same outline
level as 2 and 3. Is there any way, other than manual numbering, to
achieve that result? I'm using Word 2010 Beta but I believe it doesn't
really matter here.

Thanks.