CDDL is the license that most sources of ZFS code is under. And Reactos is GPL is not compatible.
Now a few things will Reactos except non GPL drivers. As long as the drivers are under a open source license. Even if license is not directly compatible with GPL.
Also header files and other incompatibility. Is there anything in the reactos header files that would cause trouble for a ZFS file system driver using CDDL.
Same kinda of assessment are kinda required to get other company's to provide open source drivers as well. Because not all company's use GPL compatible licenses for there drivers for legal reasons.
If mixing is not allowed starting from base documentation would be required. That is not exactly the most effective way but if that is the way it has to be done that is the way it has to be done..
Peter Dolding