You can store and re-use parts. Just save the re-usable part in a separate file. Then, whenever you want to use it, push Shift+F1, navigate to the .blend file with the part in it, open it, select the objects folder, and select your object. It will then import that object and all its data into your scene. Hope that helps!
As a previous answer suggested you can import external objects or append objects from other blend files (File > Append or File > Import). Once imported / appended, you can join them to an existing object by selecting the objects you want to join and pressing CtrlJ.