Hi David,
We have couple of different options depending on your need.
1. If you navigate in the hybris marketing application to Data management --> import there you will see the ability to add fields to "interaction contacts". This is available in latest version of yMKT i.e. 1508 and but you can only add limited data types.
2. If that is not sufficient, you can always login via SAP GUI and add/append additional fields to the INCLUDE in the interaction table (CUAND_CE_IA_RT)
3. if you have lot of fields that you want add, then bring that data in a custom table, create a HANA view and add it via segmentation customization. Please note that this makes the new fields visible only in segmentation.
Thanks
-Bharath