I don't think you can do this directly using the ALTER TABLE (AT) command.
The AT command will allow you to add a new column and specify a default value, but that value has to be 'hard coded' in the AT statement itself, you can't use an expression or calculation.
What I think you'll have to do is either:
In most situations you'll find that option#1 above is quickest.
Does that help?
Thank you @DaveWellman - Option one looks good to me. Thank you so much for your quick response. I will try this. I believe this will solve my issue. Thank you so much.