what about including Bitcoin (Ƀ) and Ethereum (Ξ)?
Problem is that you can't set the mutliplier to anything less than 1, so 1 Bitcoin would have to equal £1 which isn't very realistic and probably defeats the purpose. But if that doesn't bother you, you could of course have a currency called Bitcoin that uses the Bitcoin symbol.
Is it maybe possible to take advantage of the EURO Date feature to introduce these currencies in 2009 and 2015 respectivly?
No, there is no property in the GRF specs that would do what you want.