You'll need to create a new entry in teamgear.dbf. Open teamgear.dbf with DB Commander, scroll to the bottom and press the Down arrow key to create a new line. Then just fill out the info. Information on the fields in teamgear.dbf can be found here.
Jersey .viv files go in the uniforms folder, jersey previews or "portraits" (the .big files) go in the sgsm folder directly. You should edit a saved roster, which can be found under the NBA Live 08 folder within your Documents folder. That way, you can re-load the roster with the changes you've made from the Load/Save menu when you're done.
Open bosthome.viv with whatever (I use BigGUI) and export all the files to the 'Added Jersey's' folder (there should be 5). Rename those 5 files and the .viv file to the new name (bostc78h).