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.)