At this time, the Acknowledgements, Notifications and eReceipts do not allow for additional merge fields to be added, other than what is listed. It's not possible to add a custom attribute to be a merge field in these areas.
If you are wanting to send an email that includes the attribute as a merge field, after the information is processed into The Raiser's Edge, you can set up custom merge fields and send a message using an imported list.
If you are wanting to track data for Events (such as the T-shirt size or dinner selection), you can export participant attributes to gather that information.