Is bitcoins generate-seeds.py and makeseeds.py the scripts to register DNS-seed ips?

No, not at all. Those scripts are for creating the hardcoded seed nodes list which is retrieved from an existing DNS seeder.

To add a DNS seeder, you would need to add it to the list of DNS seeders in src/chainparams.cpp. Furthermore, you need to have a running DNS seeder which requires special software. One such is https://github.com/sipa/bitcoin-seeder.