I think I understand.....
You can't do it directly, however, you can add a "User Input" cell next to the calculating cell...
Then use an IF function to determine whether or not a user has entered any data into the input cell.
You could do it via VBA as well, however I like to stay clear of it whenever possible.
the function would look like this....
A1 is the Calculated Cell
A2 is the user input cell.
A1: IF(A2="",{calculation from other cells},A2)