How to add NOT NULL columns?

I am starting to use database-projects and struggling with simple tasks. If I would like to add a new column which is NOT NULL, how can I make it work if there is already data in the table?

I used to write diff-scripts, in such a case I would have written something like:

-- ... ADD COLUMN X ... 
-- insert data into x
-- ... ALTER COLUMN X NOT NULL

How would I need to handle such cases with a database-project?

Thx for any tipps sl3dg3