When you create an offset alignment, the dialog box will not let you input 0 for the offset width. However, after the offset alignment is created, say with offset 10, you can select it and open "offset parameters" in the ribbon, where you can change the offset to 0, under "nominal offset" top right. Thus, in your case you can create an offset alignment with offset 0 using this method, and then you can add widenings as needed. This will ensure that your start and end point both have an offset of 0.
Note however, that (as far as I'm aware) you cannot begin or end an offset alignment with a transition region. It must start and end with a parallel region. From your attached picture, I cannot tell if your alignment begins at Point A and ends at Point B, or if it extends further in both directions. If it starts at A and ends at B, you will have to create a parallel region 0.0001' long at both ends before the transitions.
