I want a SQL query will be able to run in PHPmyadmin that contributes a custom area (having a value) to any or all existing posts. Can anybody help? Is even possible?

This can be done by placing in to the postmeta table utilizing a choose query around the posts table.

Within the example below I personally use wordpress_ as my table prefix, your setup may vary.

place into wordpress_postmeta (publish_id, meta_key, meta_value)

choose ID, 'my-key', 'my-value' from wordpress_posts

Yes, you could do, but it's inadvisable. Your computer data might be lost next time you upgrade. If you are not thinking about improving, then you are departing gaping security holes inside your site. The suggested method of doing this is by using the postmeta table. This is exactly what it is available for.

EDIT

Since I better comprehend the question, disregard the part above. See comments for additional particulars.