You can do a full system import only. If you intend to use a project import (as suggested by "merge" in your question), then you will need to upgrade your 5.1 data to 6.4 before exporting it, such as by setting up a new 6.4 instance and importing it there, then exporting it from there to use for the project import.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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