Custom attributes

[Nananana]

Hi.
I have a general-purpose system.
My question is about 2 of the most basic tables:
1. Users
2. Messages sent to users (The UserID is a FK)

I need to add attributes to the messages, but the system has a common
base (for all customers) and some custom attributes:
- Some customers will need Policy # and Claim #
- Other customers will need Reference and Expiration date
These are just examples. I can't tell about the future requirements
for the custom attributes.

Where would I put them?
The easiest solution is to put everything in the "Message" table. some
columns will be NULL. It will eventually become loaded with garbage.
How can I manage the custom attributes in another table? Keep in mind
it should be as generic as possible.
Thanks in advance.