|What you are asking for is a perfect example of a Circular Reference, which is not allowed in Excel.|
To state your question another way, what you essentially asking is "Can I use =SUM(B1+A1) in B1."
Try that and Excel will pop up the Circular Reference error message since it can't resolve a formula based on the value in the same cell as the formula.
Perhaps if you explained your ultimate goal, and why you want do what you're asking, we could come up with a better solution.
It may involve VBA, but I don't want to go down that path without some more details.