i did a search a while back it is possible if you want to spend a lot on external pcie slots they do cost a lot atm for pi's , i wanted to know my self so i went looking some have done it but the cost may not be worth it unless you have the know how , tools and time to do it . i think i found some external pcei express stuff that might work but at 500 to a 1000 dollars a pop not worth it. if i had saved the link i would post it for you but i didn't after i saw the cost.
it had some thing to do with arduino
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here is something on it someone has all ready looked in to on the forums , what i found cost more and was able to be used on a pi type .!!! . im not gonna brother right now i have to rebuild my farm slow .so for me it's cheaper to build it with mother boards etc but it would be nice to use PI's it's cheaper and might even work better .
https://bitcointalk.org/index.php?topic=1551013.0