Thanks for trying HDP.
I think it is possible to integrate the 2 monitoring systems:
Here are my 2 points. Feel free to try.
1. The HDP nagios installation has configuration files under /etc/nagios. Please refer to hdp nagios configuration files accordingly.
2. The HDP plugins are under /usr/lib64/nagios/plugins/. Please try to copy those plugins to your plugin folder.
Let us know if you have any questions.