i hope that day would come realy soon! btw, you can add 1 and 1 object into your editor, i have already added some buildings to mine not with Samps object ofc, but it is posible to do, i will recommend that you add one and one object from Samps objects, becouse if not you will destroy a bit of MTA editor, i did that once to test and the "Delete Buttom" got destroyed and many another objects got destroyed that following the orginal San andreas map. so this is a downside by using Samps objects.
you can add Samps objects, but its not usefull, you have to replace current objects in MTA.
I hope that one day, MTA will let you add your own cars, objects, weapons, peds by not replacing them but really adding new things to the game