You can download RPMs ad later can install it. But some time you will face dependencies issues.
Lets say, you want to use flash plugin, you can download it from adobe site(RPM) and later install it…but not sure about any dependencies. This is just example.
and if not Google <software> openSUSE and you may find it in a community repository.
Whenever you download software from a repository such as packman or videolan which is not part of openSUSE the repository will normally be added to your list for updating purposes.