Coolant mixing / topping up

[pothole]

Need to add a little coolant to my '97 Box. Existing coolant appears to be a blue/green colour. Assume that makes it the older OAT green coolant rather than the newer HybridOAT stuff. Anyway, I'll be replacing all the coolant along with the water pump fairly soon, but in the meantime, can I mix in any OAT compatible coolant? It probably only needs a very small top up, 1/3rd litre, maybe less. Also, given that the coolant will be flushed long before winter, might it be better not to risk coolant incompatibility and just chuck some distilled / filtered water in?

Thanks