Merge a table into an existing one I know how to JOIN two tables as create a new table out of the results, using CREATE TABLE result_table AS ( SELECT * FROM tableA JOIN tableB ).

I forgot to include some columns from the JOIN in the result_table. Is it possible to merge some of columns from tableA into result_table? Is there a performance improvement over simply dropping and re-creating result_table again?
You are better off dropping and recreating. Why ?
ANALYSIS
Let's take the hypothetical example from your question
CREATE TABLE result_table AS ( SELECT * FROM tableA JOIN tableB );

This is literally a pure Cartesian Product.
Let's say tableA has 2,000 rows and tableB has 5,000. A Cartesian product would result in result_table having1,000,000 rows.
Now let's say you added the column col_1 to tableA.
If you had to fill in tableA into result_table, what would the join look like ?
First, you would have to graft col_1 into result_table.
ALTER TABLE result_table ADD COLUMN col_1;

That's a pass through 1,000,000 (1 million) rows to inject a column.
Then, you have to perform a join of the ugliest kind
UPDATE result_table JOIN tableA
ON result_table.column_key_from_table JOIN tableA.column_key_from_table;

Remember I said joining 2,000 rows to 5,000 rows made result_table 1,000,000 rows ?
To perform the UPDATE JOIN, you would have to create a temp table with 2,000,000,000 rows. Yes, 2 billion rows (1,000,000 times 2,000). The temp table has to be created. Then, you have to perform pass through the 2 billion rows to populate the col_1 column.
CONCLUSION
I think you want to be finished working with the table so you can have time to raise a family, collect your pension, etc.. So, I'd go with the dropping and recreating the table.