How to get an existing Lot into the Lots and Houses Bin, without deleting the original.
I like some of the Maxis lots, and often wish I could use them in other neighbourhoods. It is quite easy to package an existing lot so that you can install it to the lots and houses bin. You can use this method for any lot- including those you have built yourself.
FIRST,BACK UP YOUR FILES!
Go in game and choose your lot. I have chosen 88, Road To Nowhere, Strangetown.
All I need to do is click on the 'package Lot' symbol, the one that resembles an asterisk on top of an arrow.

Choose 'Package to file', and after a short pause a pop-up will appear to tell you where the packaged file is saved.

Exit the game and go to that location- usually My Documents\ EA Games\ The Sims 2\ Packaged Lots. Double click the Sims2Pack that the game just created.
You will notice that, along with the lot is packaged every recolour of every object you have in game, plus possibly a few hacks (if you have them). You do not want these to be reinstalled to your game, so what you need to do is click the 'Select lot file only' button on the Clean Installer window, so that the top item- the lot itself- is the only thing selected.

Click the install button, and install to the default location- this will be the teleport folder.

Now close the Clean Installer, and load up the game. Enter a neighbourhood and check out the Lots and Houses bin.
Your lot will be there, and you can now place it in any neighbourhood.
Important Note: If you are packaging any of the inhabited lots, for example The Smith's house in Strangetown, you MUST NOT use this method. Doing so will corrupt your new neighbourhood.
You must make sure that you select the lot segment only when you install the Sims2Pack of an inhabited house. Installing inhabited lots into the game is known to corrupt neighbourhoods and cause gameplay problems, due to the memories and associations that are packaged along with the family.