Probably not the perfect solution, but a program which will do this nicely with some major additional features would be MindManager from Mindjet.
They have a free evaluation version, I personally use it for project management, but there are many who implement GTD with it.
Actually you can also get an add-in called ResultManager for MindJet which adds many GTD functionality to it. They offer a free evaluation as well.
I like mindjet as it works well with MS office, and actually will export an outline to MS Word. As well it offers many ways to integrate with other programs as well. Anyway might be a bit much for what you want but worth the try.