You can't today.
The reason it works this way is for consistency - some "MOD columns" doesn't have to be "mapped" to one single database columns but to its property. Eg components, user types with multiple columns and so forth, all use one MOD column even though the audited data is (can be) represented as multiple columns.
If you want to, you can report your issue here https://nhibernate.jira.com/browse/NHE