Can you add onto the existing run-in as someone else suggested? Can they do the work? I bought two run-ins and left a 10' gap between them. I then just had to slap a back wall and roof and have another "bay".
If the BO/family can do most of the work themselves it won't be as expensive as I'm saying.
If she's not running the barn as a business then it doesn't make sense to incur a substantial cost to accomodate a boarder.
What are the alternatives? Can your horse be brought in during bad weather? Can the others be blanketed? Can the existing run-in be modified (overhang, extension)? Any new run-in will have to be big enough to accomodate the other three, b/c a bully will take the biggest section for himself. (The bully here hogged the 10 x 24 run-in, while the three horses that liked to share crammed into the 10 x 10 space mentioned above!).