These corp email address on exchange have a BES? You would need a separate BB for each one of them unless unless there is a different route like making it go through pop and add them thru bis--not sure of this one but I doubt it...just thought I would put it out there.
Another option is to use GroupWise with BES and use Exchange with BIS / OWA. Its the best you'll really be able to do. ... but then I wouldn't knowingly recommend someone take a HH off of an Exchange BES and put it on a GW BES.
There is a third party solution that is kind of costly, Notify link. We use it for some of our high end users that wanted to do the same thing. One nice thing about it is that it completely separates the 2 email accounts.