There is a way but I'm not sure I can actually tell you- I will try though! :)
If you go to any books page, like this one-
http://www.goodreads.com/book/show/64528...

right underneath the description you'll see a very light gray 'find at' and then some links should be there. GR uses default links, I think amazon and a few others but you can change that. I use BookMooch, paperbackswap and amazon myself. Now I just have to remember how to change it and let you know.
Okay- right after those links there is a link you can click on in (I think) dark green that says 'more options'. If you click on that it'll take you to a page where on the right side you'll see an area that says 'find this book at:' and then across from that you'll see 'customize order'- click on that and you'll be where you need to be. On the right side of this new page there is a list of all the sites you can use and a link that can 'add' them. And on the left side you can put them in whatever order you want them to be at.
I hope this helps- I hope this is what you wanted. I use the feature all the time and it comes in handy a lot. There is probably an easier way to get to where I told you to go- possibly through settings or something but this was the way I remembered and found. If you need any more help let me know. :)

I like to check bookmooch when I stumble across things I'd like to read... do you know if there's a way to cross-check my "to-read" list with bookmooch (other than actually GOING to bookmooch, I mean)?