I have seen comments on this forum before of people using a Fiberfix for a long time, but I think 1500 km would exceed that.
If worst case, you need more spokes and can't buy your size, buy some extra long ones and cut the heads off. Then try to do the Z bend thing. I have never tried that but if you have a narrow enough pliers, like a heavy duty needle nose, you might be able to do a Z bend. Spokes do not like to bend very easily so it would take a lot of strength to hold a pliers for this to work, a vice would work better than a pliers.
I did a google search with this forum name and "Z bend spokes", here is one link but they are mentioned in many other links, you could find more if you do a search:
https://www.bikeforums.net/bicycle-m...ire-wheel.html
Good luck.