to add objects of any type (also bushes) without using any addon (even left out the editoraddon) there's one more or less simple way.
First, get the references for the o.pbo's from the ofpec editor's depot/tutorials.
The part with the bushes you'll also find
here.
Then open up the editor. For the first try i suggest you use a empty map.
Insert an object of your choice, the danger sign does a well job but any other will do also.
Then SAVE THE MAP!
Open the mission.sqm of this mission. Find the section with the danger sign.
I'm not at my own pc so i cant give 100% correct info but i guess you'll find it.
There must be a line saying
name = "Danger Sign"
or something similar.
Now just replace "Danger Sign" with the name of the bush you'd like to insert. In the o.pbo reference they're listed with the .p3d extension. Just drop the extension. Then just save the mission.sqm
IMPORTANT!
Don't doubleclick on the object once you changed it, since for the editor it is still the danger sign and therefor will overwrite the changes you did.
But you can move it around, turn it (press shift and move the mouse) or copy it (ctrl+c/ctrl+v)