Custom data in p12 certficate

Is it possible using openssl to generate a personal certificate with a custom field (customerID) and related data (1234) and get at that data in apache / webspeed

At the moment I am using SSL_CLIENT_S_DN_EMAIL to store the customer ID and would prefer not to as I want to use it for the email address

I suppose I could use SSL_CLIENT_S_DN_EMAIL_1 (multiple email addresses) but still looks yukky.

Thanks

Julian