You can save you creations (copy and save for posterity or later importing).
eg export the new bridge
When importing you need to edit the object you want to place the objects inside, and paste the export text then save.
All of the exported objects you pasted in, end up inside the containing object.. so make sure its something you can easily get into (I forget if we have a command that transforms an object into an doorway to itself) so for simplicity build an import box. edit it
Notes about the export format: the obj_id number is only used when linking objects, this is just a reference so you can not overwrite existing objects that may have the same id.
The level indicates how far down the tree obj objects it is.. larger numbers live inside the preceeding smaller number.
Note: If you re-edit and save an object which already has import text pasted in.. you will duplicate everything all over again!