No, you can't. I tested various things with transformation file when it was released.
data/db/shapeshiftcommon.xml
Things i've attempted, with only the shapeshift file and not using something like race.xml.
Basic collect, meaning you have the mob as soon as you reach the appropriate rank. (like pumpkins and wild horse are basic) Can't add mobs.
ShapeRaceID, the first value at least can't be altered. For example, the seal is "ShapeRaceID="(580002,100)". Altering the 58 part should be what changes the appearance, but it does nothing.
EnableSkillList would allow you to add or remove skills for the creature. Adding skills will prevent the icons from going dark, ie like when you're on a horse the icons darken, but you still can't use the skill(s) that you edited in.
ShapeShiftLock is what determines which transformations can use the emotes. Changing that has no effect.
Bonus stats, various ones in here but they determine what is boosted by the mob you transform into/collect. You can't adjust the values.
Assimilate Rate, ie collection rate/chance. If you change a 10 into a 100%, it won't do anything in game. Changes the text value shown on the collection sheet, but does not truly give you a 100% collection rate.
Finally the assimilate complete count, ie how many points you're gonna need to collect to transform into a mob. Things like bats would be 5, while lich would be 1. Decreasing the value from 5 to 1 won't affect gameplay. Like the assimilate rate, it will change the appearances on the collection sheet though.
Some of the mobs will have a Scale Factor. It's why when you transform into something like the dragons, you are actually smaller than the original dragon. Altering this does nothing.