Hello Emilio,
You cannot configure SLA monitoring on the FWSM at this moment.
Maybe in the future this great feature will be added to this modules.
I know the 6500 supports it so you can try to set it up there.
Regards,
Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC