Mapping multiple discriminator values to single default class in NHibernate

I have an existing RoleType table with data. I am trying to map this table in NHibernate using Table per class hierarchy:

Here, IRoleType is an interface, with implementors RoleA, RoleB and RoleC. This works. But here's the problem -

The table contains rows with "extra" discriminator values (2,3,5,6) that are not mapped to a persistent class. These values are deprecated in the domain, so its not useful to create persistent class for each. But we also cannot delete them from the database.

Is there a way to map these extra rows to a single "default" class? If not, how else can I solve this problem?

Thanks!