In countries where bitcoin is legal, they must have considered as an asset. So you can follow the usual route to add them to your will similar to what you are going to do with your other assets. Unless Bitcoin is classified in a different way, I don't see a reason why you can't do that.
You will face challenges only when Bitcoin is banned or not legally recognised in your country.
Actually, in my country, bitcoin is not legally recognized. I wanted to know how it is done in the countries where the use of bitcoin is legal, from there I can deduce how to approach it in my country. It seems that in the next 4 years, many countries will be accepting bitcoin as a means of payment. US has set the baton rolling.