Yes, but you have better time buying separately on MaxiZoo or Zooplus because the dimensions are not right on galaxus.

I have seen that bonnet can be bought separately, but basket is in set I think, but here is a promotion :
KlickFix Doggy Basket (40 l)

This purchase includes the following items:

-The basket (strong for me but the dog must not weigh more than 7.5kg in my opinion)
-Weather protection (incredibly useful)
However, you'll need to buy the bike adapter separately.

I have only had the basket for 1 week and therefore cannot answer the question.
I don't know whether and what you can buy separately.