KB and farmer walks are mentioned in Forever in Assistance stuff, single leg/core, so they can be done in the workout. Farmer walks are NOT recommended as a conditioning tool, tho, the recommendation is 240-400m total of walks and the weight doesn't need to be substantial - I like doing them at the end of the workout.
On the off days, I like doing KB swings. 10 sets of 10 reps EMOM works good, the only kettlebell I have (52lbs) became quite light for me so I can pretty much do 10 sets of 10 reps back to back superset with band pull aparts with little to no rest between sets, works good for active recovery.
I guess sledgehammer works a similar way but never done it so can't give you feedback.
Keep in mind that conditioning has to be taken into account when you run the program. For most people, 2 days of hard conditioning (KB swings, sledgehammer, running, sprinting fall into this category) are more than enough.
Some programs allow for more heavy conditioning, others require for you to hold back on it - it really depends.
If you're in doubt, I guess it would be wise to do one day less of hard conditioning, and one day more of easy conditioning (walk, weighted vest walk, airdyne and such)