I notiiced thatFFG S.W. uses "Strain", is there a way to combine fatg. and stress for WFRP to reduce the accounting?

The easy answer that comes to mind (without having seen the Strain rules in S.W.) would be to add toughness and willpower and let that be the threshold. All effects which would add or remove stress or fatigue would be counted as strain instead.

Each point above the threshold adds a black die to all rolls, regardless of which characteristic they are based on. This is to minimize counting how many black dice would be needed for each and every roll.

Going above your threshold prompts you to recieve a temporary insanity. You pass out at double your threshold.

This is just and idea from the top of my head, never used it. In our group fatigue/stress work well as they are.