If you just want to add an additional column to the array, you need to re-dimension it. Using your sample...

DIME aMyarray[2,10]
aMyarray = "ABC"

...gives you two rows with ten columns each. To make it two rows with eleven columns each issue...

dime aMyarray(2,11)

...doing so will add a column to each row. However, row 2, columns 10 and 11 will be initialized to .f.

HTH