Welcome to the forums, David! Yep, you can upgrade from a 1-person license to a Family Pack. Just head over to
our online store, then click on OmniFocus. Click the dark blue "Upgrade" tab on the next page, then click the "Buy" button next to "Upgrade to: Family License".
The next page will ask for some info about the license you purchased from us previously; you'll have access to the upgrade price ($40) once it's been entered.
(The Mac App Store doesn't offer upgrade support: if that's where you purchased our license, email
our sales folks and they can assist. Off the top of my head, I'm not sure how they handle that case.)