can you use Tabulate in your report?
You cannot (easily) do cell merging in table box. Like hogi did already ask, could you just use the tabulate in your new window/report?
Most of the things you can do with table box, can be found from Scripting Index

Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Car Poll.jmp");
nw = New Window("",
H List Box(
Distribution(Continuous Distribution(Column(:age))),
tab = dt << Tabulate(
Show Control Panel(0),
Add Table(
Column Table(Grouping Columns(:sex, :marital status)),
Row Table(Grouping Columns(:country, :size))
)
)
)
);
Scripting Guide > Display Trees > Construct Custom Windows > Examples of Creating a Dashboard from Two Reports
There are also some options outside of table box (such as using lineup box) but those generally do require more scripting.