There is Qt documentation on using CMake with Qt. There is similar documentation on the KDAB web site.

There is an (abandoned) SourceForge project which supposedly generates CMakelists.txt files from .pro files, but as the comments there imply, it may not work well for complex Qt projects.

It would be better from a maintainability point of view to just pick one build system or the other and port your projects to use that one exclusively. Otherwise, I think anda_skoa's suggestion to run cmake from QMake (or the corresponding cmake version to run an external QMake process) is the only way to have a mixed build system.

A mixed build system would probably give you headaches trying to track down why (and where) builds fail, because the two build systems probably won't communicate problems to each other very well.