You're probably going to need to use a trigger to change the text to lowercase as there's no way to indicate on a column that you want values stored in lower:
CREATE TRIGGER TriggerName ON TableName
FOR INSERT, UPDATE
AS
IF UPDATE(ColumnName)
UPDATE TableName SET ColumnName = Lower(ColumnName)
To clarify, using a more real world example for table and column names:
ALTER TRIGGER trg_lowercaseEmployeeId ON Employee
FOR INSERT, UPDATE
AS
IF UPDATE(employeeId)
BEGIN
UPDATE e SET e.employeeID=Lower(e.employeeID)
FROM Employee e INNER JOIN inserted i ON (i.ID=e.ID)
END