Take a look at \Move or Die\Lib\Sound\Announcer\ for examples.
The Lines.pdf file seems to contain a list of all of the lines you need to record, but compare against the existing annnouncers to be sure you get them all.

Create a folder like
\Move or Die\Mods\MyAnnouncerMod\Lib\Sounds\Announcer\MyAnnouncer\
Repalce MyAnnouncer with the name you want to put on it.
Add your custom ogg files to it.

And be sure to add the thumbnails and the info file in the \Move or Die\Mods\MyAnnouncerMod\ folder. See http://www.moveordiegame.com/modding/intro for more info on those.

i found that the other@mystery lines dont only play when an unknown gamemode is selected, it plays when any call without a designated line is played. for my pack "the mixtape announcer" i made lines for every game mode (even blowback [RIP]) and made the menu@go voice line non-existant o that it chooses from my massive list of random lines.
P.S. you can add as many lines for each scenario ad long as they are numbered name@example_X