Hi,
there is no standard extension with diffuse functions for the def2 basis sets.
You can take the flat functions from the aug- basis sets out of the basis set library
and add them manually in the basis file. But you should know what you are doing
in that case.
For most 'standard' applications the diffuse functions don't really improve your
results, but they make the calculation a lot slower, because integral prescreening
will not have an effect anymore. It is probably best to test basis sets on small model systems.
Cheers,
Arnim