If the file you want to import is not a kGTD document, make sure it has the hierarchy you want and has a column for each kind of value you want to import (title, start date, due date, and so on).
From the File menu, choose Import.
Choose the OmniOutliner file you’d like to import. If it is a kGTD document, OmniFocus automatically figures out how to import it properly and you’re done. If it’s an ordinary OmniOutliner document, you’ll have to help it along a bit; carry on to the next steps.
A column mapping dialog appears. For each column in the original file, choose an OmniFocus column to receive the data.
All of the data is imported as a single new project. If you want the top-level items of the outline to become their own projects, just select all the contents of the new project and choose Edit ▸ Outlining ▸ Outdent.