I’m reluctant to encourage blending two different ideas or programs. It doesn’t always work out the way you hope, or as the program is written.
I suggest either doing WSSB or putting together a full-body routine similar to TTT, SFM, The Next Big Three, Ripped, Rugged Dense, etc.
Although you may be able to use some of the ideas of WSSB in a three-day-a-week full body routine, I think it will also ruin its effectiveness.
I, personally, think it’s best to use full-body routines or split routines (upper body/lower body). So following WSSB would fit in with that.
As for the extra leg day, this is what Joe DeFranco recommends:
A. Unilateral Movement - Perform 3 sets of 15 reps each leg
*Choose from the following list of exercises: (Don’t do the same exercise that you performed on your first lower body day.)
Single leg squats, back leg elevated
Barbell step-ups with knee lift
Barbell reverse lunges
Barbell reverse lunges, front foot elevated
Barbell reverse lunges, front foot elevated (with knee lift)
Low-pulley split squats, front foot elevated
Speed-skater squats (1 and a half rep single leg squats)
B. Posterior Chain / Hamstring movement - Perform 3-4 sets of 10-15 reps for TWO of the following exercises:
Seated or standing good mornings
Stability ball hamstring lifts
C. Weak Point Training - Perform 1 exercise for 3-5 sets of a lagging body part
(Common areas include upper back, external rotators, abs, grip training, etc.)