I create my date dimensions manually, but my hunch is the wizard will not help you at this point. You will need a script to add the date values. As you are adding information, there is no way to avoid reprocessing the dimension or all the cubes attached to it (which might be all of them).
Sorry, as that's not what you want to hear, but you just have to dig in.
BTW it may be what the wizard gave you for a name, but DimTime usually refers to hours and minutes. DimDate is a better name for a dimension that includes dates. I don't use either, but it helps with consistency.