Fireworks CS6 - coverting images to squares and centered via batch

I am having a difficult time converting many web images into what I need, so requesting some help.

I have many disparate product web image sizes, ranging anywhere from 30 x 400 to 325 x 375 for example. I am trying to convert these into 400 x 400 squares for my new website, and having the old images center both horizontally and veritically in the new 400x400 size. The new images would maintain their current pixels, just looking to add padding on either X or Y axis as filler to make the final square image.

I have tried creating various new commands and scripts to no avail. Typically the images will be off-centered or somehow cropped so they aren't converting properly.

Any assistance in creating the proper sequencing for a command that could be used in a batch run would be greatly appreciated.

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