To do such an export i installed the livestatus broker module in my classic Nagios systems and made a livestatus query to get all hosts from the system. This is now some years since the last time a needed to do this. I don’t know if there is a broker module available for the actual Nagios 4. For Nagios 3 it should be no problem as these broker module is also used inside the CRE.
With this you could also export the group assignments. But i don’t know a way to automate this then for an import.
To get a good overview what you can do with livestatus you should consult the manual here.
Small guide how to install livestatus inside an Nagios4 on Debian
In the end livestatus is the key to migrate config data from an classic Nagios to the outside.