How do I do this? in the VirWox official page it asks for the name of the avatar in Second Life...
You don't need a registered avatar at SL (just fill in any name) to exchange USD to SLL and SLL to BTC. The system seems to be very secure and the company behind it is well established.
but take care: You will only be able to buy full BTC amounts (no cent amounts) so there will always be some unused SLL left in your virwox account. If you don't have a registered SL account you won't be able to use the amount of SLL left in your virwox account.
So try to deposit the exact amount of USD/Euro to virwox that you will use to exchange to SLL and then BTC.
Anyway, I found a website that does the same thing (allowing me to buy linden dollars and then trade them with bitcoins), that one, eldex exchange.
https://www.eldexchange.eu/en/news/index.html
But it has the same problem, when I want to register to the site, it asks me for the name of the avatar wich seems to be required.