I'm not 100% sure if you can set n or "location" when setting specific values, but values will get added to end by default (I think).
Below is example from Scripting Index. It will add new row with name colum set to "David" and age column set to 15.
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
dt << Add Rows({name = "David", age = 15});
Also see this Scripting Guide > Data Tables > Rows > Add Rows
-Jarmo