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Are you tring to add content to a module or to a wysiwyg editor?
For the first:
Yes it's possible. There are 2 ways of accomplishing this:
1- Without use of smarty templates
In this case open the corresponding php file (for ex: article.php), perform your db query and output the result [echo $var;]. Be advised that this method could break ajax functionality if it exists in the module.
2- With smarty templates
This method stats the same as method 1, but instead of using an "echo" you will insert a new smarty var. This requieres changes in 2 files; the php file and smarty template.