It is not possible with org-mode tables. However, have a look at table.el package (included with emacs for some time so try: C-h d table). Org-mode has some support for tables from this library, e.g. when exporting, but don’t expect full compatibility.
As for simulating merged cell, it depends on what you want. Inlining text strings in the same cell might be enough for computation/publication, but not for visualisation.