You can not - that is one of the several reasons that referenced libraries are strongly discouraged in iPhoto
What version of iPhoto do you have? Do you have Aperture? Do you have iPhoto Library Manager - http://www.fatcatsoftware.com/iplm/ - ?
Well, strictly speaking, you can do this.Two ways. One is tricky:
How are you at editing SQL databases?
This user found a way:
But tread very carefully - get it wrong and you'll hose everything. Make a back up first.
The other is expensive:
Purchase Aperture, and if you have the latest versions of both these apps, then you can open the Library with Aperture, and use the relocate Masters command.
Or, in the end, make a managed Library. A Referenced Library is more work for zero gain.