Here's an example that uses Big Class. I make a subset table using row 8, and then append it back to the original Big Class table.
dt1 = open("$sample_data/Big Class.jmp");
// Make a subset using row 8
dt2 = dt1 << Subset( Selected Rows( 0 ), Rows( [8] ),
Selected columns only( 0 )
);
// Concatenate the new table back to big class
dt1 << concatenate(dt2, append to first table);