If I'm not mistaken, that will require code and I'm a rusty on my Excel VBA, order of events, etc. I think something like this would work.

Add a button. When pressed, run the code:

Check value of A2 and store in variable, A2Value.
Recalculate
A2=A2+A2Value
End sub

Perhaps repost this in the Excel Programming forum. I'm not sure if it will get any more looks here now that I've responded and bungled it all.