In principle the setup would look like the following.
- your check host in the other network creates output like an stored snmpwalk from CMK
- this output is then transferred to the CMK server and stored as an stored walk for the host to be checked
- the host itself on the CMK machine is configured to use a stored walk
- you have to check that the stored walks are with an actual timestamp
- the check interval should be between 1.5 and 2 times the interval for the new data from the remote machine
Yes it is much to do 
This should be no problem as your license covers all checked services in your environment, you can have instances as many as you want. All services together must be under your licensed services.