You CAN do anything you want. Obviously extra sugar isn't helpful for any but the most healthy/active people. Do you have excess fat? Do you have metabolic disorder? Depression? Struggle with cravings and hunger? Do you lose weight easily or not so much? That banana has a heck of a lot of sugar already (for many people even natural sugars are problematic) so I wouldn't think adding more is beneficial. I've been here almost 3 years and can't consume a fruit smoothie. I'm better off with eggs, meat and veggies... or just BP coffee.

Based on your Food Diary notes today, I'd suggest not to add sweet to anything and also that fruit smoothies probably aren't the best right now if you are craving/bingeing. (I had BED and avoiding extra sweet really matters.) Some strawberries in coconut milk is a nice snack option though.

Recently I've been omitting the banana and making smoothies with 8 oz almond milk, a quarter avocado, 1-2 tbsp raw cacao powder, sometimes protein powder, 1 tbsp ground flax seed, and a quarter cup of blueberries, plus a little water to adjust the thickness. If I need a little sweetness, I use stevia or 1 tbsp of pure maple syrup. Yum!

OP - I will echo what has already been said in regards to your smoothie in that I would think the banana and strawberries likely made it sweet enough. In other cases where I want to add sweetness to a recipe I only use honey (local & raw if available/convenient) or maple syrup. I don't use stevia so I can't speak on how good/bad it is.

It's so true. I used to add butter and maple syrup or brown sugar to sweet potatoes. Now they taste plenty sweet to me with just butter.