Select the BF image, Select All, Copy, Add Slice, Paste (repeat for number of slices in fluorescent stack).
Process>Image Calculator>ADD, create new windown, image1: BF stack Image2: fluorescent stack> process all images.
That should do most of what you want. Of course you can do this in a macro, where nSlices() gives you the size of the fluorescent stack and a for loop is a convenient way to duplicate the brightfield image.
Maybe you need to preprocess your brightfield image and your fluorescent stack so adding up the pixel values yields an acceptable end product image.
If you prepare a slightly bigger destination stack, you can even correct for the image shift by moving the selection when pasting.