Growing them out first is a good idea.

I don't know if having the steep sides is good for turtles. I think they dive down to the bottom from time to time.

I'm just guessing here, but if they have what they need to thrive in your pond, they might stay, or even if they leave, they might always come back.

I can't see any reason not to try it unless these things cost a few hundred bucks or so.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."