うわー、わかりました...さらに多くのGoogle検索が明らかになりました:
Texture2D tex = (Texture2D)renderTarget;
ただの単純なキャスト:)
これが私の最終的なコードです:
public static Texture2D Crop(Texture2D image, Rectangle source)
{
var graphics = image.GraphicsDevice;
var ret = new RenderTarget2D(graphics, source.Width, source.Height);
var sb = new SpriteBatch(graphics);
graphics.SetRenderTarget(ret); // draw to image
graphics.Clear(new Color(0, 0, 0, 0));
sb.Begin();
sb.Draw(image, Vector2.Zero, source, Color.White);
sb.End();
graphics.SetRenderTarget(null); // set back to main window
return (Texture2D)ret;
}