Not a problem at all especially if you have an IN with a TA.
Have them change your reservation to a Full. (might be a Cancel/New behind the scenes but only a slight change for you-your original reservation should have had 1 night fee paid as a deposit - since a Full is cheaper than a Preferred, the NET effect on the deposits would be a small refund to you). But temporarily you would pay a Reservation/deposit for the new Full reservation and wait for the refund on the Preferred (takes a few day - up to a week)). They should be able to add a request - when I've call the WDW reservation line and asked for a request, the CM took it over the phone and I've gotten it EVERY time I checked in (or an apology that went like this - "oh I see you requested 900 loop but there is no space available there today. Would you like to stay in 1000"?).
I've never used a TA because there is no advantage that I can see to using one. My dates are pretty set most times, there are NO discounts for the Fort when I've traveled there, and I can control my reservation for sites, meals, activities. (If you call to change a reservation and you don't "own" that reservation (not who's the guest but who can make add/changes/deletes), you're out of luck).
Bottom line: a TA should be able to move your Preferred reservation to a Full, make requested loops for both cabin and site reservations, and be able to link them together. Disney CM's can and do and a TA sees the same system the Fort Cast Members do. If your TA doesn't know how, they can ask around and make it work for you. Because they are working for you.
Bama Ed
PS - our family did multiple cabins on past trips. The big family vacations are a lot of fun but it's like herding cats trying to make anything happen at a certain time.