Update:
To produce a table that includes the collapsed letters report, simply include "Connecting Letters Report ( 1 )" inside the "All Pairs" portion of the code to make the Oneway chart. See below...
NamesDefaultToHere(1);
// Get some data
dt = Open("$SAMPLE_DATA/Typing Data.jmp");
// Add a 'By' variable with 2 levels to dt (results are repeated)
n = NRow(dt);
dt << Concatenate(dt, AppendToFirstTable(1));
dt << NewColumn("By", Numeric, Nominal, Values(VConcat(Repeat(1, n), Repeat(2, n))));
// Do an invisible Oneway analysis
ow = dt << Oneway( Y( :speed ), X( :brand ), By(:By), Means( 1 ), All Pairs( 1, Connecting Letters Report ( 1 ) ), Invisible );
// Save the 'Connecting Letters' information for all levels of the 'By' variable
owRep = ow << Report;
dt2 = owRep[1][TableBox(5)] << make Combined Data Table;
dt2 << set Name("Connecting Letters");
owRep << close Window;