The stats are not close enough
Legend uses a lot of familiar D20 terms and ideas, but the way it calculates its values is vastly different from how 3.X does so, as is the way it measures power and difficulty. If you tried to just import a monster, there'd be all manner of problems.
But there's good news!
A monster's pre-established stats or concept can give you a great start. Legend is a modular system, designed for ease-of-use in mind, and if you can distill a monster's concept into a few words ("Melee brute" "Hit-and-run flier") you can probably make something that's recognizably close pretty easily. Using a gargoyle as our example, we can see that it's a flying monster with multiple natural attacks and an inclination towards stealth. Dragon and Demon both provide the monster-to-be with claw attacks, and Dragon does flight too, so we pick out the Dragon track to use. Do we want our gargoyle to be fast? Add Acrobatic Adept. Hit with the weight of a crashing boulder? Add on Path of Rage. Once you've determined what tracks, if any, need to be used all that's left is to decide if you want to stat the monster up like a character (making something that's more individually powerful, since the only thing it doesn't have that PCs do is Consumables) or a Mook (which results in a weaker, more expendable creature).
The short version: pre-established monsters from D20 systems are great inspiration, but you'll still have to make the monster yourself.
For Your Convenience
Did you know that you can fill out the Legend sheet provided on the site by typing into the various fields on it? I didn't for awhile, but it can make doing the grunt work a lot easier, and it does some of the math for you to a limited extent. Since you're just statting monsters, typing them out on the sheet PDF and printing them could greatly smooth your process.