Is it possible to include one HAProxy backend in another?

We're running multiple sites on the same hardware, and we'd like to serve them using the same pool of application machines. However, one site periodically gets greatly-increased traffic, and so we'd like to reserve a small section of machines for only the other sites.

Is it possible to do something like this in HAProxy?

backend foo
    server app00 0.0.0.0 cookie app00 check
    server app01 0.0.0.0 cookie app00 check
    server app02 0.0.0.0 cookie app00 check

backend bar
    server app03 0.0.0.0 cookie app03 check
    include foo

I realize that we could simply define the machines in multiple rules, but I'd rather avoid duplication if possible.