It is possible.
Depending on what is set in your Options > Write, i.e simple or advanced controls, you can edit the edit-form.php or the edit-form-advanced in the wp-admin folder, by inserting your text in the textarea div. (Or you can do it in both files.) In the advanced form it would be around line 100.
I am not sure I understand your question.
On the other hand, now that I am more familiar with WP I wouldn’t recommend altering the core files… I’d rather go with a quicktag solution.
See the tutorial about how to do this: http://www.tamba2.org.uk/wordpress/quicktags/