Frankly maps for TTT that don't have a ttt_ prefix are wrongly named. The game has no other way of telling TTT maps from other maps. The Fretta vote can't list every map, because you'd also get all the maps from other gamemodes (like sandbox) in there.
As said by Tommynator, you can manually work around this problem by adding the map names to the fretta_maps part of info.txt. For example,
"fretta_maps"
{
"1" "^zm_"
"2" "^ttt_"
"3" "^dm_"
"4" "^cs_"
"5" "^de_"
}
Would become
"fretta_maps"
{
"1" "^zm_"
"2" "^ttt_"
"3" "^dm_"
"4" "^cs_"
"5" "^de_"
"6" "^clue"
"7" "^alcatraz"
}
And so on for other maps. This way Fretta knows it should list those maps too.