Hi Lovisa,
I've never seen anything that will allow you to copy the forecast from one item to another automatically - not even with the new forecasting tool in R3.
Liked your thinking to use the Formula to create the demand. Was thinking about a way to put it on the Formula AND use it as a supplementary item. Can you think of a way to put it on the Formula such that it would create demand, but without actually consuming it? Possibly depends on how you create your picking lists journals.