Is it possible to alter/add a product in a customers order?

My products are all downloads. Each product contains 2 parts - an info file that is set to be included in the download automatically, and then file with the designs that the customer purchased. For this file there are a choice of several formats to choose from - and although a warning message pops up that a format is not chosen, some customers are indeed able to complete their transaction.
When it then comes to the download page - only the info files are available for download and not the actual design files. Of course this leads to much unhappiness for the customer.
What I do then is to email the files to the customer, but ideally would like to be able to insert this file to the original order so that it will be available in the customers order history.
Is there any way that this can be done?