Cool Cat: this may help. I found this on the .mac help page
Originally Posted by Cool Cat
Upgrading to a .Mac Family Pack account
If you're a full .Mac member, you can upgrade your account to a .Mac Family Pack at any time.
A .Mac Family Pack includes one full .Mac membership with 10 gigabytes (GB) of combined email and iDisk storage, four .Mac sub-accounts with 2.5 GB of combined email and iDisk storage, and a shared iDisk folder for you and your family.
Note: .Mac Family Pack sub-accounts cannot add email-only accounts or upgrade storage.
To upgrade your individual account to a Family Pack account:
If you're not already logged in to .Mac, go to Apple - .Mac, and log in.
Click Account in the left column of the .Mac homepage, and then confirm your login.
On the Account Settings page, click Buy More.
Choose Upgrade from the Family Pack pop-up menu, and then click Continue.
Review your billing information and click Continue.
Review your order and click Buy Now.
Wait while your order is processed (it only takes a moment). Don't use your browser to view another page until you see your purchase confirmation.
After you purchase a .Mac Family Pack, you can convert a .Mac trial account or an expired .Mac account to a Family Pack sub-account. For more information, search .Mac Help for "Converting a .Mac trial account to a Family Pack sub-account" or "Converting an expired .Mac account to a Family Pack sub-account."