Let me list some approaches that can be used for integration:
sending events into netxms using nxevent / nxaevent command line tools
pushing DCI values - nxpush / nxapush tools
netxms agent can monitor a log file and create events on matching lines
web services can be used to get data from other systems - it's also possible to use nxsl script to parse received jsons.
some external app can connect to netxms via Web API or nxshell
