Use HEX editor, for example:
You want rip STAR FORCE game:
- download Star Force rom file from any rom site
- open HEX editor and see first bytes of Star Force PRG rom data and CHR data
- open Dumped multicart
- use SEARCH option in HEX editor and search Star Force PRG and CHR data first bytes
- Mark/Cut/Paste Data into new file (Join PRG+CHR)
- add header
any other games in multicart - similar method