Edit Deformers > Blend Shape > Add >
If the blend shape deformer blends target object shapes in parallel (the In-Between creation option was off when you created the blend shape deformer), you can add the new target object shapes so that they work in series with one of the existing target object shapes. One quick way you can identify the appropriate value for Target Index is by looking at the order of the target sliders in the Blend Shape Editor (Window > Animation Editors > Blend Shape). Note that in the editor, each target object shape has its own target slider. In the editor, going from left to right, the Target Index value for the left-most target slider would be 1, the next 2, and so on.
If the blend shape deformer blends target object shapes in series (the In-Between creation option was on when you created the blend shape deformer), Target Index can only be 1 because there is only one target slider. In this case, you don’t have to specify Target Index, but you do need to specify the In-Between Weight.
Specifies whether to check if the added target object shapes have the same topology as the base object shape and the existing target object shape(s). For example, if using NURBS objects, you could check if all the shapes have the same number of CVs. Click Check Topology to turn it on or off. Check Topology is on by default.