Although I haven't verified this in CE, in EE 6.2 the Configuration ->
Control -> Nagios main configuration section allows you to specify
Misc directives, which could include cfg_file=xxxx.
DassIT has done a lot of work with APIs that access Monarch, the
underlying configuration system to Nagios in GroundWork Monitor.
DassMonarch is their SOAP-accessible API. GroundWork Monitor includes
DassMonarch without the SOAP access. I would be surprised if they
hadn't thought of how to hook the Addon up to GroundWork Monitor using
the DassMonarch API so as to avoid messing with files.