Now just copy your object's .shp file from you userlib folder to your terrain texsrc folder. The next time you open the TE the new object should be in the list of available objects that you can use.
Cheers,
asw
Woops - forgot a step in the OE. Select the shape you've imported into the OE and give it an applicable property (bombable, structure, etc.), then click on the "add" button. Save it (file->save) and exit the OE. The new shape will be in your userlib folder.