You can use the linux command pwgen to generate usernames. With the right arguments, it generates almost pronouncable names of random syllables.

pwgen -A01 [<length> [<amount>]] is the general syntax. -A01 makes it so that you get all lowercase without numbers and one username per line. The <length> parameter lets you choose how many characters your username should have and the <amount> parameter lets you choose how many usernames it should suggest.

Example: 5 usernames with the length of 10 characters:

$ pwgen -A01 10 5
aefaegeina
avubeixaik
shienaitie
jaethuekeg
yoreivoozu