[BroZeus]

For this you need to loop through all user files OnGameModeInIt and load data and createvehicle.
BUT sadly currently it is impossible to get listing of files/directory through the natives that current SA-MP provides, for achieving this you need a file management plugin which supports directory/file listing functions
Here I found a list of File management plugins from Plugin List topic :

• FileFunctions v0.1b by RyDeR' - https://sampforum.blast.hk/showthread.php?tid=252918
• [REL] FileManager 1.4 (24th of June 2012) by JaTochNietDan - https://sampforum.blast.hk/showthread.php?tid=92246
• y_files - YSF file functions branch by ****** - https://sampforum.blast.hk/showthread.php?tid=177028
• RL_DIRS - makes your life easy by RyDeR' - https://sampforum.blast.hk/showthread.php?tid=411268
• Virtual File System by Xentiarox - https://sampforum.blast.hk/showthread.php?tid=364876