I hope you remember back to the days when 200MB/day was an incredible amount to have. :-)
There are some simple things and then some more complicated things.
Limit access by time of day
First of all, you can block times of day for connections by MAC address (advanced settings) on the Airport Extreme base station. This can keep machines from pulling down files, updates, etc., without your permission.
Cache data with a proxy server
Second, you can install a "caching" proxy server, such as Polipo http://www.pps.jussieu.fr/~jch/software/polipo/, which will allow multiple machines to access the same content, fed from your iMac instead of the Internet. You'll need to perform the same sort of proxy chaining as you currently do with GlimmerBlocker. You should look around for a caching proxy that offers plugins for ad blocking, etc.
Limit your bandwidth so you cannot exceed your daily transfer limit
Third, you could perform what is called "rate limiting", "traffic shaping," or "QoS" (Quality of Service) (see http://en.wikipedia.org/wiki/Traffic_shaping). You can do this by installing software (http://intrarts.com/throttled.html is one I have Googled but have not used) or by purchasing a home router that offers this option and putting it between your AE base station and the satellite router. Here's a list from CNET: http://reviews.cnet.com/routers/?filter=500563_5554972_
With this option, you could do some rough math to figure out, given your peak usage, how much bandwidth to allow to your network. I don't think the software solution will work with your iMac proxy solution since people do not connect through the iMac (but the caching proxy will help) so you may have to spend $50 or so on a router and configure the allowed bandwidth.
This will basically cause you to treat your 200MB/day satellite connection as a 18kbps modem assuming 24 hours a day usage. If you really only use it eight hours a day, you could have the equivalent of a 56kbps modem. Fudge up or down based on your comfort level. It will stink but you won't have overages, and you can always "turn it off" if you need to make a big download.
It will also allow your entire family to download video or other media. It will be self-correcting, in that it will be so painful to download high definition video that it won't be worth doing.
Best Answer
In theory, it is possible. Practically, it's not worth the bother for most, if not all, home users.
In order to fully combine the bandwidth (i.e. without having restrictions on single connections), you would need to have a "partner" server on the Internet that you can communicate with trough your local ISP and through your cell connection. That partner proxies all traffic to/from your network - essentially combining the data from both internet connections and sending it on as one.
For that to work you also need a specialized network device on your home network that is placed in between your local network and your router and the cellular connection. It splits up traffic and sends it on to the proxy server (and vice versa for incoming traffic from the proxy server).
All in all, it requires specialized knowledge about networks to set this up, it requires additional equipment on your home network, and it requires a server on the internet. This is usually relatively costly to come by, so it's not really worth it to add a smaller amount of extra bandwidth to a home user connection.
Larger companies (as well as smaller companies specializing in something that requires specialized network access) implement something somewhat similar by the way of BGP routing. This allows having multiple paths to the Internet, and to be reachable through multiple paths at the same time. However, it also requires you to have your own autonomous system number (AS-number). It is free, but takes time to apply for. As well as requiring your to have your own set of routable IP-addresses (usually PI for smaller setups, and PA for larger). These IPs are getting harder to come by. Usually you would need to have at least a /24 (i.e. 256 IP-addresses) in order for this to work at all - probably even more today. If you have a use case where IPv6 only is enough, the requirements are a lot easier to fulfill. However you still need to have corporation from both ISPs in order to make it work.