No, you are not getting it wrong, there is a limit of 10 items to a rule object. You can specify a network range instead of a network address if the IP addresses are in blocks.
There is an API that may allow you to feed your data into NSM, you should talk to support and see if it is possible. The API guide is only available from support.
Your other option is to look at the integration guide and see if you can utilize any of your other applications to improve performance.