Sounds like U have the basic idea. Also keep in mind that the car that U are importing must be in the form of an ini file. For instance, the one U linked in your second post is a zip file, which must be unzipped first before U can import it. Here is a description of how to do it once U have the file unzipped, (so it's in ini form).
1-Start your sharkport software, wait for the "waiting for command" screen.
2-Click on "mem card" button, (bottom left of PC software).
3-Check off the box next to your GT3 save, and click the "backup" button.
4-Now click the "PC Database" button, (bottom center of PC software).
5-Locate the save and check the box next to it, click the send button, select file, and pick a folder to put it in.
6-Now open MK's, click "Read Garage", and locate the save and select it.
7-Hilite a cell in the car U wish to replace, click "Import Car", and locate and select the ini file for the car U want to place in the save.
8-Click "Write Garage", and give the save a new name, and save it to a folder.
9-Go back to the Sharkport, in the PC Database, click "Import Saves", locate your new save and select it.
10-Locate the save in the PC Database window, and check the box next to it, and click "Restore. A screen should come up asking if U want to replace the existing save, click yes.
Here's a few other things
1-I deleted all of the saves in my PC Database window, so it is empty. This way it's easy to locate the save that U import.
2-Keep in mind, that once U import the save into the PC database window it will not have the name U gave it, but instead will be named what ever it was when U downloaded it from your mem card in the first place.