Hello,

This is a great request. I would however like to point out that you can achieve this with Custom Client Fields. Our documentation outlines this here:
http://docs.whmcs.com/Custom_Fields#Client_Custom_Fields

I do however see the added benefit of a built in option for this, with an associated report.

Staff update:I've released a blog post regarding this feature request:http://blog.whmcs.com/developers.php?t=65894