I don't believe that you can. I seem to recall that you can go the other way though. Have you made sur eyou have the latest point revision for FCP? That would be about your only option.
Why are you wanting to go back and forth between FCE and FCP?
If its simply to apply a filter or something, you can always render it in FCE, save as a quicktime reference movie ( still in DV ), and then open that resulting file in FCP, and add whatever you wanted to do in FCP.
it's a huge kludge that will involved alot of unnecessary rendering, but I think that would work.
You can load your FCE files into FCP fine, it's just that you will need at least FCP 5.1 to be able to read FCE 3.5 files. Just remember though, once you load a file into FCP it will be converted to FCP format and will no longer be usable in FCE.