the "Problem" is that in OLAP terms any value in a cube is identified by its dimension elements. So if you could "just" add a dimension to an existing cube there is no defined point of intersection for your values anymore.
To "add" a dimension to a cube you can export exisiting cube data, delete and rebuild the cube with more dimensions and import data again. At that point you will have to decide where the value should be saved in your new model.
You add an Dimension "scenario" to your cube which contains the elements "act" and "bud". While importing the former values you have to decide, whether it is a "bud"-value or a "act" one.