If the custom agent monitor does only basic monitoring, then you need not make any changes in the existing monitor entry point.

#!/bin/sh
# CustomAgent Monitor script
. $VCS_HOME/bin/ag_i18n_inc.sh
RESNAME=$1
VCSAG_SET_ENVS $RESNAME

# Logic for custom agent resource monitoring.
# Based on logic set STATE to "OFFLINE" or "ONLINE" 
if resource is found in either OFFLINE or ONLINE state.

if [ ${STATE} = "OFFLINE" ]
then
	exit ${STATE}
fi


if [ ${STATE} = "ONLINE" ]
then
	exit ${STATE}
fi