Like others say, the shell limits you to 260 and the file system (normally) has a much larger limit.
JRiver performs a sanity check on import (on Windows only) and rejects filenames longer than the shell can safely support. This is done because we don't want to create other problems later (why can't I find my file in Explorer, why is my sidecar not working, etc.).
I think we might someday remove the JRiver safety check on import. After that, we would handle any length file but not create a filename longer than 260 characters ourselves. We would make this change sometime when we were happy to deal with possible problems, which won't be in the near future.