Creating Seed Address - Direct Mail

All, 

I need a little help. I have a Direct Mail client that would like to add seeds to their file before exporting to the Print vendor. Their data layout has over several custom fields, and their seeds are in the same data layout as well.  Their seeds have specific data that needs to be populated - and some occasions left blank - so we want to make sure a "random substitution of fields with data" from the live file does not occur. 

Because their campaigns can contain anywhere from 40 - 75 seeds, I would like to create a subfolder(s) under Seed Addresses, load all the seeds under that subfolder, and then be able to use the "Edit the dynamic condition" under Seed Address tab to pull the seeds in during deployment. 

My question is - because of the custom data fields, do I need to extend the Seed Address data schema to do so, or can I add the custom fields under the "Additional Data" tab, and when I load the file, it will populate the seed record based on what I loaded? 

I was hoping to use the seed option in Adobe vs loading the seed list and joining it with the live data prior to export. The Seed Address documentation seems sort of fuzzy to me, and I feel it talks more towards email vs Direct Mail. 

Any guidance would be appreciated.