I am not quite sure what you mean by "additive cell evaluation". Do you want to always replace a given input cell by its output -- thus deleting the contents original Input Cell? If so, Here is an example of a single button Palette that will do this:
CreateWindow[
PaletteNotebook[{
Button["Evaluate In Place",
Module[{nb},
nb = InputNotebook[];
SelectionMove[nb, All, CellContents];
SelectionEvaluate[nb, None];
SelectionMove[nb, All, Cell]],
Method -> "Queued",
BaseStyle -> {11, FontFamily -> "Helvetica"},
Appearance -> "Palette"
]
}
]
]
--David
http://scientificarts.com
