To export (clone) a Sim using SimPE - this saves out the Sims looks - bone structure, genetics, that sort of thing, not his personality.
You need to use SimPE - the latest version for the EPs that you have installed - get the latest version from http://sims.ambertation.de/
Select Tools (top menu).
Select Neighbourhood in the menu.
Select Neighborhood Browser.
Choose the neighbourhood where that Sim resides and Open it. This can take a while.
Then go back up to Tools/Neighbourhood.
Select Sims Surgery.
Find your sim. In the "Patient Sim" field at the top, click on "Use".
Once you do that the word Export above that same box where you see the picture of the Sim should now be in blue.
Select the blue Export and save the Sim - it is safe to change the file name at this point. I put the Sim's name into the file for ease of reference so, for example, I just made N002_User00018BellaGoth.package.
Your Sim will now be in your Saved Sims folder and should in theory show up in both BodyShop and Create-A-Family Screen. She can be recreated in the game, and if you set the personality points accurately, she will be an exact replica of her other self.
Why would you want to do this?
It's a safe way to move a Sim from one Hood to another without messing up your Hood files by bringing in "memories" of all the Sims she knew in the first Hood. When humans move from town to town they generally lose touch with their old friends too...
It is also a good way to rescue a Sim from a Hood that may be already failing.
More SimPE stuff
Many thanks to SimsHost Hullabaloo contributors, TSR contributors and others in Sims community for explaining these things to me, sometimes repeatedly :)