You can use \PassOptionsToPackage{OPTIONS}{PACKAGE} to set additional options which should be used when the package gets loaded. But you have to use it before the package gets loaded.

Or you could use PreventPackageFromLoading from the scrfile package, which is part of KOMA-Script. You could prevent the package from loading and later load it yourself after you reset the prevented packages with \ResetPreventPackageFromLoading. More in the KOMA-Script Guide (scrguide).

Set natbib options after loading natbib package

  “I meant to give each of you some of this to take with you,” said she,
  “but as there is so little toast, you must have it now,” and she
  proceeded to cut slices with a generous hand. We feasted that evening
  as on nectar and ambrosia; and not the least delight of the
  entertainment was the smile of gratification with which our hostess
  regarded us, as we satisfied our famished appetites on the delicate
  fare she liberally supplied. (Jane Eyre)


What’s the meaning of ‘on’?
