How do I not lose/replace files when syncing an iPad with a PC that hasn't been synced before?

I have a photo album on my computer that I want to sync to the iPad. I have synced this computer with other iPads and this iPads with other computers before, hwoever, not the same one. How do I transfer/sync the two devices so that I don't lose/replace any files currently on the iPad? I merely want to 'add' files from the computer to the iPad.

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