As stated above you need to integrate those accounts directly with BIS (and not receive their messages via your gmail account) if you want to reply using those accounts on the BB.
As a workaround you can integrate the accounts and set their default filters to not forward any mail. However, you'd have to use Forward instead of Reply when replying to those messages and change the Send Using field to the desired account.
Alternately, you could filter email coming through your primary gmail so that none of the other accounts are delivered to the BB through the primary gmail account and let BIS retrieve email the way it needs to (separately through each account). That would allow you to use Reply normally. Though if you're BCC'ed on email the filters probably wouldn't work properly.
Originally Posted by billpoly
I really hope that BB will support respecting the original To address when replying at some point, but for now I'll manage.
I suspect it needs a way of validating that you're authorized to send mail on behalf of those accounts (to avoid allowing anyone to spoof any other email address) and without supplying BIS with the credentials for that account there's no way for it to know. I wouldn't count on it ever being possible on the BB.