If the current contents are green, it's regular old glycol. Add as required, no big deal.
If it's not green (Yellow, orange, red) chances are it's a long life option. There's much dread spread online about the disasters that will result if you mix long life with regular glycol, but it's from days of old and isn't really an issue anymore. I wouldn't recommend doing it if you can figure out what exactly is in it and refill with the same, but it's not going to result in the molasses/gunk disaster that you may read about online, still spread around by many even though it's no longer a thing.
Either way, although completely draining and flushing is never a bad thing especially if you don't know the strength or age of the old coolant, but topping up isn't inherently bad.
Topping up with water isn't a great idea. You are lowering the freezing point of the coolant, and unless your bike is stored in a heated place during the winter...if you get that freezing point too low bad things WILL happen. You'd also need to run the bike up to temperature to draw that water out of the coolant reservoir to begin with and fully mix it..if you don't, this time of year, the reservoir will probably freeze and crack. Always use a 50/50 mix of glycol to water - for the small amount bikes use just buying premixed is a simple solution.
