Unfortunately if you do want to buy stocks with Bitcoins right now , you have to first convert them in the local currency and then proceeded with buying stocks normally like you do with money .
Problem is :
1. Government and companies see Bitcoins as a Competition and therefore allowing people to trade in Bitcoins regarding the stocks wouldn't be profitable for them.
2. Bitcoins is in no way related to fiat , bank accounts in many countries ban buying and selling of cryptocurrencies , conversion of Bitcoins does take time and at the same time the softwares right now are not equipped to handle this simple task , no one is interested in making them .
This is undoubtedly easy , but it does involve added risks. There will be concerns regarding the privacy.
The fee will double and at the same time it would take a lot of time (time is gold in the stock market) . Plus people who are interested in stock market and people who are interested in Bitcoins , it might seem the same but they are on two opposite poles .
3. It is much easier to actually just buy stocks and Bitcoins separately without hampering your privacy
Maybe in the future they will allow the same but right now you should keep your Bitcoins secure and try using the traditional way , till we have a solution.