I can't see any way to get extras_require to be installed, unless some other package indicates an extra feature in the package that needs to be supported.
For example, if you want HTTP support in Paste, you can install WSGIUtils. But this isn't required. How can I get WSGIUtils installed? Or, more specifically, there's a bunch of examples distributed in the Paste source distribution; these examples require lots of extra software, even if Paste doesn't. How can I easily get all the examples working?