Ok, strange requirement. Yet, you can use
TRY-CATCH in SQL for it.
Try something like:
BEGIN TRY
SELECT
MyNewColumnNameToAddIfNotExists
FROM
MyExistingTable
END TRY
BEGIN CATCH
ALTER TABLE MyExistingTable ADD MyNewColumnNameToAddIfNotExists nvarchar(50) DEFAULT 'Free' NOT NULL
END CATCH
P.S.: You might need to make sure that the error is because of missing column only!
Further, if needed you can try Transactions along with Try-Catch. Look for details
here[
^].