How to Export project that could be used in open-source SoapUI?

Unfortunately as far as I am aware, there's no way to automatically make a project ready to use in the free version of SoapUI. What really decides whether or not you can is whether or not you have used any functionality that only exists in the Pro version of SoapUI. If you haven't, then the project should be usable as is. If you have, you would have to find a way to change your project to not use such functionality.

For example, for my projects, I store a lot of my properties in a properties step and can access them across the entire project and various test suites with property expansion. This does not work the same in SoapUI free/community edition, so to achieve the same affect I would have to store them at the test suite or project level as custom properties.