You are correct, and I'm sorry, I didn't notice that point in your original post. I'm afraid, then, that what you want to do is probably not possible. If you can't use code, and you can't use qRules (because of Admin Approved template issues), I don't believe there is going to be a way for you to realize your desired functionality.
Any chance that simply displaying the secondary data source info in the form without copying it into the main data source would be acceptable to your end users?