To write in DITA for learning purposes, and then convert back to unstructured Frame for PDF and CHM generation, requires two tools and a modest investment of time.

One of the tools we're using is the FM_DITA Application Pack beta recently released by Adobe. With this, you can develop DITA topics, map them into documents, and then generate a book from the map.

The other required tool is a utility to convert the book files from the FM-DITA template to another template. Whether such a template conversion tool is commercially available, I don't know.  We developed one in-house to help us convert our legacy documents after we substantially redesigned our template.  If anyone is interested, I'll explain how it works.

The FM-DITA Application Pack has given us the opportunity to write a few DITA concepts, tasks, and procedures, and assemble them into maps. We chose a modest pilot project for this. After creating the material in DITA and converting it to our standard template, we had to make some adjustments. That's where the small investment of time comes in.  For us, this meant reformatting in a few places and adding cross-references (we haven't tried out the DITA reltable feature yet). The result is a new document that conforms to our usual requirements and is ready for our standard processes. Meanwhile, we've gained experience with DITA.