InDesign can't split cells. You'll have to add another row, and fake a
spit cell manually. If that's too much of a pain, a script could do the
fakery automatically. But even with a script, it's impossible really to
split a cell.
Yes indeed, it's a limitation of InDesign that cells cannot split pages or columns. It's a PITA.
Do you know how to find a scrip that does it?
Do you know if this feature is somewhere in Adobe's roadmap for next releases?
Re scripting a solution, see Ariel's response # 1.
Anyone who knows anything about future releases would not be at liberty to tell you.