Hi
Welcome to Centrify community!
Actually the Modify certificate prompt is the same as "Add". Therefore, please go proceed with the following steps:
1) Once downloaded the p7b file
2) Login with an account that has administrative privileges
3) Double click on the p7b file, a password prompt will appear and ask for admin account credential
4) Fill in the password and click "Modify Certificate"
5) Afterwards, you should be able to see the certificate added into the Keychain.
Hope this can answer your query. Please keep us posted with any update or result. Thank you!
BR,
Ivan