You can append to a multirow, but in this case I believe you should write one query that returns all the rows you will need.
Doing the seperate query for each package_id will be inefficient. I believe the results of the get_content_items procedure may be cached, but it still seems like the wrong way to go about it.
Another option would be to allow get_content_items to accept a list of package_ids or have it append to the multirow name that you pass in if the multirow already exists.
