(1) Ways to stay "unaffected" if the domain registrar's servers (NOT just the domain) themselves are DDOSed, if any.
the registrar's servers only matter if you are using them for DNS (or hosting or other services, obviously). Once your domain is registered, the records go into the root registry and you don't need your registrar to be on line for your domain to work. If they are your only DNS provider then you want to consider adding more than one.
(2) "How to have more than one DNS service provider for a domain?
(for this part you do need your registrar online, so you can enter the changes through them)
In your domain registry account, add multiple authoritative DNS servers hosted by multiple providers.
This will probably require NOT using the registrar's DNS service so that you can enter the 3rd party servers. (eg with godaddy you can't use their "domain control" in addition to 3rd party providers, you have to choose "my domain is hosted elsewhere" to set your dns)