I am not sure exactly what you are saying...you can place regular HTML in a text field, which could be interpretted upon display. Or, you can take a forum like approach and have the input use special tags, such as and whatnot.
As for just including a seperate file, that is also possible. Then, in the DB, just have a varchar field that stores the path or URL to the file you want to include.
What you can do really depends on how much time you want to spend implementing the system to do it.