Adding new table doesn't require downtime at all.

If your queries are not using SELECT * then you are ok to add new column (as it will not affect your application functionality). To check locks while adding new column, you should add column in your test server & see the result.

Adding column will lock the table.

A table, as a whole, has a single schema (set of columns, with associated types). So, at a minimum, a schema lock would be required to update the definition of the table.