Although you can convert content to DITA manually, you can also convert your HTML content to DITA through the XHTML to DITA transform in Oxygen.

Converting content to DITA isn't a small undertaking, because you'll essentially have to retag everything with the DITA markup. There are some automated ways of converting content, but if your source content isn't already in a DITA friendly format, for example, if you have lots of topics that combine lists and concepts, or that have nested subsections (third-level headers), the conversion might require some restructuring. Nevertheless, you can speed up the process using a combination of HTML Tidy and Oxygen's XHTML to DITA transform.