I'll give you a bit of a tutorial if that helps. I'll do one for ferns, and it's basically the same for all of them, just find the name so you spell it right.
1: open object maker
2: open FeralHeart - Media - objects - shrubs (the files on your computer, not the game)
3: In Object maker click on Mesh and type fern1.mesh
4: down the bottom there are six buttons. Click add new
5: and there is your fern! Now, click the button at the top that says object
6: Now, type in the name (we'll use fern) and the Group (we'll use MyPlants) and don't forget capitals when typing the group.
7: Right beneath The line where you write the group are 2 buttons, save and clear. Click save.
8: Your fern should now be saved! To check, click on MyPlants in the scrolling area labeled groups, and then search through the scrolling area labeled objects for your new object
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This should work with all .mesh files in the objects folder.
Hope this helped!