The "How to Ask" page seems like a really good place to put guidelines for new users, and a general explanation of what to try for with your first questions.
But the text seems to be fixed. Is there any way us mods can edit it and add more content?
If I remember right Seasoned Advice got theirs changed to reflect that recipe requests were off-topic. This was at Joel's suggestion and implemented by SE corporate. An e-mail to someone on the team or other communication seems like the best bet, unless one of them happens to check in here and see specific suggestions. Also possible would be to suggest a modification to the SE sites to allow this feature.
So no, mods cannot change the text. However the text can be changed.