Hello mainrotor,
Start off as Brommee9 says by selectred the first cell in an empty column and typing ="US" & A1 ... assuming row 1 is the foirst row.
Now drag the formula down to the end of column A row
Now select the column B data
Select the first row in column A
paste special values
Delete column B or wherever the formula was, column A now has the prfix hard typed.
Regards,
chris_bottomley