"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.
Just an absolete product. I have used the cheapest AMD based boards with simplest 30$ APUs with modern in those days AMD HD78xx GPUs. All that stuff was installed on steel shelf without any boxes, using PCIe extention cords. It was several years ago and it was profitable. I am not sure that right now you could earn enough on mining.