Sorry, there is no tutorial to use libopencm3 on System Workbench.
Maybe you can try to:
1/Create a new C Project > Makefile project (Ac6 STM32 MCU GCC toolchain).
2/Select your target
3/Copy the sources into empty project then add your code then build it the Makefile.
I think it will build the libopencm3 static libraries.
If so, you will be able to create an empty Executable project (without firmware) then link the library by selecting the libopencm3 project in your Project properties > C/C++ General > Paths ans Symbols > Reference.
I’m not sure it will work easily because System Workbench is dedicated to development on STM32, the project are configured for the ST low-level libraries.
Hope it helps a bit.