This is a display problem, not a conceptual problem.
The black cells with numbers don't really exist conceptually. They're just convenient places to put the sums.
Display-wise in Excel, I suppose you could divide each Kakuro cell into nine smaller Excel cells, arranged 3 x 3. You could put the vertical sum in r3c2, and the horizontal sum in r2c3, of this 3 x 3 arrangement. The nine Excel cells comprising each Kakuro white cell could be merged into one.