Apparently you can spawn vehicles from /tt races into /race mode by going into /tt, choosing a race and right before the timer starts the countdown (that is, not letting the timer start to countdown) do /race and press tab to spawn the vehicle of the /tt race you chose.
I could not do this bug myself though but I saw other players achieve it ([LSR]Draguna and [SC]Merc).
Must be right timing or something.
Note*** they were not abusing this bug. just doing it to show me it could be possible.