And here's a generalized script, closer to Excel's Paste Special > Add.
- Select source cells.
- Command-c to copy values to clipboard.
- Select destination cells (must be the same number of cells as source cells).
- Click triangle 'run' button.
SG
set pasteItems to (the clipboard)'s paragraphs's items 2 thru -3
--> puts cell values copied to clipboard into an AppleScript list
tell application "Numbers"
tell document 1 to tell active sheet
set t to first table whose selection range's class is range
set selRng to t'sselection range
if (count pasteItems) is not (count selRng's cells) then return
repeat with i from 1 to (count selRng's cells)
tell selRng's cell i
set its value to (its value) + (pasteItems's item i)
end tell
end repeat
end tell
end tell