Okay, if you don't need the original sources to be updated with the additional data you enter in the large file, then the problem becomes very easy. You already have a union query, right? Then the next step is to open a new query, add your union query as the data source for the new query. Select every field. Now convert this new query into a Make-Table query. There should be an option on the toolbar to let you do this...if you have trouble finding it, consult Help under Make-Table. Once you run (!) this new query, you will find a new table has been created with all your data from the union query. You will be able to enter additional data into this table. However, none of your original tables will reflect the additional data...as I am understanding your post, this is okay. If not, let me know, and we'll look for a new approach.
Hope this helps...good luck!