"The B150 chipset limits the second PCI-E x16 slot to a PCI-E 3.0 x4 connection."
Bandwidth doesn't matter when bitcoin mining. You can run 5 cards on that board without any penalty.
Here is an example of an adapter that is commonly used for these types of systems: https://www.amazon.com/Powered-Extender-Litecoin-Bitcoin-Mining/dp/B00HZ0M9F2/ref=sr_1_2?ie=UTF8&qid=1472596744&sr=8-2&keywords=pci+express+x16+adapter
Here is a supporting article: http://cryptomining-blog.com/1153-checking-out-the-pci-express-x1-x16-usb-3-0-extenders/
With that said, you wouldn't want to run SLI/Crossfire on this system. Mining however is a different story.