Using Lag() (and possibly check for first row) should be enough here.
Names Default To Here(1);
dt = New Table("Untitled 5",
Add Rows(3),
Compress File When Saved(1),
New Column("Column 1", Character, "Nominal", Set Values({"", "aaa", ""}))
);
dt << New Column("Col", Numeric, Nominal, Formula(
Row() != 1 & !IsMissing(Lag(:Column 1, 1))
));
-Jarmo