I suggest to add a connected project. You can do that with menu Tile Mapping/Settings…
A connected project shares the same tile library and you can have different maps there (frame = map), ground, background objects etc. . Pick up the objects from this second project using “Tile Pick & Place” mode, go to you level and place it there.
Once you have them placed in your level map you can also pick from there easily.
The connected project settings are much improved with the next version. Currently you first have to create a second project having the same tile size and add it to the other project with the menu entry mentioned above.
(Use page up/down buttons to quickly switch between projects.)
Hope this helps.