NPM/NTA

I think you should be able to do this with a Network Discovery job.  Probably the easiest way to do it would be through a bulk upload by IP address or hostname.  The Network Sonar job won't duplicate the node if it already exists in the database, but you'll get the option to select which type of interface you'll want to import.  Just select serial interfaces and it should add all of them to your routers.  If you have multiple pollers, you'll probably want to run one job per polling engine, as you can select to which polling engine the devices will be imported. 

I attached a screen shot of a test discovery I did showing the Network Sonar job skipped a node that was already in the database, but it added an interface that wasn't being monitored. 

Hope this helps!