I think,,, you cannot do that, but you should export JPG with a fixed focus first. Then you can edit the JPG file. Needless to say, iPhoto cannot directly handle light-field pictures (i.e., the file format of Lytro pictures) unless Lytro provides a sort of plug-in software for iPhoto.

and is adding such a plugin feasible?

Stefan, it is feasible but reasonably complicated.  We would love to have an iPhoto developer create a plugin, but we need to do more work on our side first.  We will also be working on adding our own editing tools.

Have you been working on it since August?  Lytro is great but is really limited in sending photos...iPhoto is faster and easier to use