tutorial: https://www.youtube.com/watch?v=860YxRdeAj0
showing off what's possible (flying vehicle, the Elephant is apparently still out of our reach): https://www.youtube.com/watch?v=8ih9Tjk5WDA

I have tested this and seen other people test it and it works.

The steps aren't obvious but they're fairly simple:

1. Spawn the vehicle you want.


2. Spawn the objects you want to weld to it.


3. Set the vehicle and the other objects to phased physics.


4. Merge the objects into the vehicle.


5. Select all the objects and make them a group.


6. Make sure the vehicle has a respawn time long enough to where you can go through some menu options before it respawns. (You can change the setting later.)


7. Make sure the parent object for the group is not the vehicle. This is crucial, otherwise the game will crash to dashboard when you try to weld.


8. Destroy the vehicle and wait for the main part of it to despawn.


9. Go into the object options of something you want as part of your creation and weld the group before the vehicle respawns. (The command is only available in that time.)


10. Wait for the vehicle to respawn. It may take longer than expected-- don't get impatient and delete things, otherwise you'll get a stationary vehicle you can't delete.


11. Set the newly complete welded group to normal physics.


12. Check that everything looks and moves like it should, save your map, and drive your creation with style!

A few more notes:

Even welding tiny little things to a vehicle will affect how it handles when you drive it, but I'm sure with practice you can get used it.

People have already made a bunch of things with this like flyable Pelicans.

Apparently trying this with a turret makes the game crash, so adding turrets onto things isn't doable right now.

The possibilities are still just... I don't have the words for the potential. You could weld something onto the firing parts of some vehicles to make "scout" equivalents that can't hurt players at long range, or make vehicles larger to more easily prevent them from getting inside buildings so you can have more open buildings, or weld fire effects onto something sticking out of a vehicle (not the vehicle itself, that'll destroy it quickly) to leave a trail of fire, and those are relatively crappy examples I just made up. This also makes me really hopeful that this will become a supported feature in the future which could mean even more possibilities and some really cool things in matchmaking; I had basically no hope for this before, but now it looks like all they need to do is fix some crashing issues and the unmovable undeleteable object issue and hopefully make it less obtuse to get working.