I don't know of any tools to make an ipr into an eclipse project, though since both are XML files, an XSLT file could probably manage it - I did do one that build basic project info from an Ant build.xml
Fortunately, other than the project files themselves, pretty much any project that was developed under IntelliJ can be made to develop under Eclipse, with no change (and vice versa). Just take the project source directory, do an Eclipse "New Java Project" operation and point it to your project sourcfe directory. Eclipse will make a pretty intelligent guess as to your source directories, so all that's left is to tell it where the output class directories are and to tag the libraries.
An IDE is no substitute for an Intelligent Developer.