Since you mentioned EX-Merge, I can safely assume you have Exchange.
The BES server acts upon Mapi events. Never tried it, but if importing via Ex-Merge or a PST file creates Mapi events you could try.
There are a few things you should keep in mind:
What is the setting for Keep Messages? After it has expired, the xxx8220;oldxxx8221; messages will be removed from the BB. Even if you set it to forever. Whenever the BB runs out of memory, it will start deleting the oldest messages automatically.
What type of user will be using the BB? Are they spending there time more outside or inside the office? Because inside the office they have their outlook.