Adding a header or footer to the aggregator2-item and all items in the RSS feed

Hello All,

I am making a module to add a header or footer(text or PHP code) to the aggregator2-item and all items in the RSS feed.
The module will give an option to add the headeror footer in any of the content types.
On doing the research, I think I have to make the module such that, the header or footer contents gets appended to the "teaser" and "body" field of the node table.
My question is, Is this the right course of action? Am I missing anything?

Thanks for any help you can give

Best Regards
Rudy