There is a way to add mounts without making them overwrite existing ones, but unfortunately that's not my area of expertise. I can only tell you how to go about editing them.
You need to extract them with the

first.
I do believe you can find your mounts in Vehicle -> DDS after you've loaded your .sah to the tool. I can't name right off the top of my head which file corresponds to which mount, but they are in order from the lvl 40 NPC mount to the 58 UM mount and the "special" mounts such as the snowboards and reindeer (if you have files ending in 09, it's an alternate coloring of the 58 mount).
They're also arranged according to race: Death Eater, Elf, Human, Vail.
They can be edited in Photoshop, but you will need to install

before Photoshop will support the image format.
From there, edit however you see fit and remember to save with alpha settings then re-import them to the saf with the data tool.
happy coloring c: