Can be done but it won't be easy.
You need to create a series of dynamic interfaces. You have one main interface where your SSID is tied to.
Next you need to create DHCP scope for your APs at each site. The DHCP scope has to be unique. You can't have APs in SITE A get an IP address from Site B. Site A DHCP scope and Site B DHCP scope has to be unique.
Same thing with DHCP scope for wireless clients. They have to be unique. These scopes are tied to the dynamic interfaces (of each site). For example, you create four dynamic:
1. Main_link: 10.1.1.0/24;
2. Site_A: 10.2.1.0/23;
3. Site B: 10.3.1.0/23;
4. Site C: 10.4.1.0/24 (it's a small site so the subnet mask is small)
The default gateway of Site A, B, and C resides in your router.
Next, the hard part: You'll need to create a few scripts (like PERL) to pull data from the RADIUS authentication server. Your script will say that all authenticated users will be plumbed back to the Main_Link dynamic interface.