I don't think there's a way, but I've never tried to do that. I know when you do a 3rd party back up with DM there's an option to not have it erase the information, there might be an option or something for that. Try the advanced restore, maybe
It looks like there's a way to do it but its not free, lol. I found a program called ABC Amber IPD Merger which allows you to merge all your ipd files into one large ipd backup. From there you can load it to your bb. Here's the download: