I can speak for importing the .csv file which should have simply two fields: IP address and subnet mask. An example for importing the following six subnets would be:
The import dialogue box will then list the subnets in a table allowing you to place a single check mark to import all, or selectively choose only some from the list.
What about a subnet name field? I just started 'playing' with Address Tracker and am intrigued by its potential. I've got a big Excel file of current address info (yes, I'm coming to the game late!) with somewhat over 300 named subnets. I tried the two fields identified by cjn and also tried a leading field with name but the name looks like it only comes in with the base network number. Of course I can update this manually but my lazy nature doesn't let me accept with without question!
I also found the import to be somewhat selective. I tried it with eight entries and it only accepted five. Not sure what that's about but more play time may lead to an answer.