Investing in stocks using Bitcoin makes no sense, because:
1. Bitcoin is likely to be a better investment
2. it's overly complicated and expensive to go fiat -> Bitcoin -> stocks, when you can instead go fiat -> stocks
I presumed that you want to invest in "ordinary stocks" at the general markets (NYSE, Nasdaq...). In case you want to invest in Bitcoin stocks traded at havelock, cryptostocks, mpex and the like, you should be aware that a large percentage of these so-called "stocks" are in fact unregulated junk bonds with an insanely high probability of scam, because many of these "companies" don't even have a valid address.
ya.ya.yo!
here we are referring to website like 1broker when you can invest in stocks/forex via cfd, not real stocks/forex, so the passages is only bitcoin-> your desired investment-> cash out in bitcoin, there is no fiat involved
and when someone want to invest in bitcoin, is obviously to have more bitcoin, aside from the intrinsic investment element that bitcoin already offer, like speculating...