Unfortunately, there's currently no "--env" type of flag, (which sounds like a good idea, and I'll look into adding it, you could use it multiple times, like
--env foo=bar --env fubar=baz
But for now, there's 2 ways to do this:
There's the job_flag "export_environment" which will duplicate the entire submitting environment, but it's a brute-force way to get where you want.
If you're building cmdline or cmdrange jobs, you can prefix the commands with the explicit "set" commands, which will be OS- or shell-specific:
Windows:
set FOO=bar && <my cmdline>
Linux/OS X (note that the default shell is 'sh')
export FOO=bar && <my cmdline>