Hi
This is going to be hectic task. If you want to store the information of new field, you need to even enhance back end database tables also. if it requires in reporting, searching , then all together it would be a different story.
Try to debug and found out what are the required tables and structures that need to be enhanced.
Sorry for giving this information instead of solution. weigh carefully pros and cons before doing this.
if you are using new versions ( > ehp1), try to login using administrator role and see if CM is available to enhance. you can try this before going manually.
I can think of one solution. may be you can analyse it if it is possible.
Try to create a z table that will have your custom field along with the fields that help you to find the transaction and attachment details.
you can add a value field on the web ui and display the value from this table.
however, you need to handle the update/creates/deletes manually whenever user attach new record.