Make an LXFML file with the bricks you want to modify (without any decorations applied), as well as a brick with the decoration you want to apply to the other bricks. Take note of the design ID of the bricks in the file (at the bottom when you click on the brick) and open it in a program where you can modify the lxfml (notepad++ is the easiest IMO) and edit the decorations part of the LXFML File to the decoration
<Part refID="*number" designID="*designID*" materials="*number,number*" decoration="0">
There should be one (or more, if you are replacing more than one decoration at once, just remember to make the two or more decors a different color or something so you can tell them apart!) that has a different decoration id. This is the decoration you want to apply to the rest of the decorations. copy the part that says "decoration="*number*" and then highlight a different line that says decoration="0" and press "replace". decoration="0" should be in the top box, and press paste, and decoration="*number* should now be in the bottom box. And hit "replace all" and close the replace window, and now press "save as" and save it as "*insert whatever you name it as*.lxfml" and remember to save as "all types (*.*)" and save it, go to ldd (or use it as a pallete) and make a new file in ldd and hit "import" and import the lxfml file and your new bricks should be there. Sorry about the long instructions, but i wanted to be as detailed as possible. If you want, I could make it for you, just tell me the exact bricks and the exact decorations you want. Hope this helped!