Apologies, to add new SNMP OID's or Variable to NMIS you will need to do some device modelling. You can find out more information here:
After that, we have lots of useful documentation on the WIKI.
Opmantek also has services to assist with adding device support.
NMIS supports a single SNMP Community string per node. It is standard practice to create an SNMP community string with the required access and restrictions using SNMP Views.
Replace the string you have now with the community string with the required level of access.
I don't mean a community string, I mean additional SNMP var polling. Say I have a host and I want to poll for the following SNMP var (.18.104.22.168.4.1.20632.5.7) where can I add that in NMIS to poll the device in addition to the basics of what NMIS is already polling?