How to Store and Edit Vendor No in IC Agent?

I have a requirement to display, store and edit the vendor no in the account identification.

Vendor No stored in venmap table. Currently, I added the custom field in the account identification. I am able to populate vendor no but not able to edit and store in the account identification. Is there any funcationality or configuration where we can edit and store vendor no. I am using CRM 7.0

Your reply is appreciated.

Thanks,

Banu