If you got the bellows from a dealer, I suggest that you contact them and see what they can suggest. I agree with Ruediger - I don't think it will last very long if you use wood filler.
Apart from being worn, what's wrong with your old bellows? Might they be repairable?
Oddly enough I was considering a very similar problem recently, with a set of Stagi bellows on Ebay (that I didn't win!) with a view to fitting them to a non-Stagi box with the same dimensions. That would have involved me finding out the thread size of the new bellows, getting some screws in that size, then opening out the holes in the concertina to use the larger screws.
Presumably you don't have that option.