Yes it's a very good idea. To ensure you work the soleus and associated stabilizer muscles make sure you stack 1.25kg plates only.
Step ups are a moderately useful exercise. If you need specialised equipment to do them then my advice is don't, they're basically identical to a lunge.
Step onto a bench. If someone's sitting on it, ask them to move out of the way first.
I'm not sure why you're doing stepups, though. Were you previously extraordinarily sedentary? Are you engaged in a knee or hip injury rehabilitation programme?
Fellas, this guy reckons he cant even squat or deadlift for fucks sake, Bulgarian split squats are out of the question. Try power walking.
Reverse lunges, walking lunges, Bulgarian Split Squats are all options for you.
Just moves, look at the "getting to know the squat thread" then watch a few vids from the YouTube channel squatrx (think that's how it's spelt).
Other than that, for a uni lateral exercise, as previously mentioned, Reverse lunges (I would suggest progressing to your front foot being on a step to increase ROM) or Bulgarian split squats (again, with front foot on step to increase ROM) are imo, better than step ups anyway.
Failing that, grab a couple of heavy DBs and do laps of your gym
If you cannot squat with good technique, you will certainly have problems with walking lunges, even without the pointless twist. You cannot perform a safe and effective lunge without being able to perform a safe and effective squat.
Get someone to teach you how to squat.