You can configure more than one package at a time to be a global package with the "Global Package Assignment" dialog. Right-clicking the "Computer Groups" container and selecting "Assign Package(s)..." will show the following dialog:
All packages with a checked checkbox are global packages, other packages are regular packages that are already assigned or need to be assigned to a group or computer. To make a package a global package simply check the checkbox, to clear the global flag and make a package assignable simply clear the checkbox.
