Custom Walker

Hi,

By using wp_nav_menu i can create this standard structure:

<nav class="header-menu">
	<ul>
		<li><a href="">Parent category</a>
			<ul class="sub-menu">
				<li><a href="">Child category 1</a></li>
				<li><a href="">Child category 2</a></li>
				<li><a href="">Child category 3</a></li>
			</ul>
		</li>
	</ul>
</nav>

But i would like to know if it is possible to create this type of structure by using the custom Walker class?

<nav id="header-menu">
	<ul>
		<li><a href="">Parent category</a>
			<div class="dropdown outer">
				<div class="inner">
					<section class="m-grid m-grid--divide-6 l-grid--divide-6">
						<div class="grid_unit l-grid_unit--span-2">
							<article class="list">
								<p>Child categories:</p>
								<ul class="sub-menu">
									<li><a href="">Child category 1</a></li>
									<li><a href="">Child category 2</a></li>
									<li><a href="">Child category 3</a></li>
								</ul>
							</article>
						</div>
						<div class="grid_unit">
							<article class="box">
								<article class="image"><img src=""></article>
										<article class="excerpt small"><span class="category">Category 1</span><a href="">The title</a></article>
									</article>
								</div>
								<div class="grid_unit">
									<article class="box">
										<article class="image"><img src=""></article>
										<article class="excerpt small"><span class="category">Category 2</span><a href="">The title</a></article>
									</article>
								</div>
<div class="grid_unit">
									<article class="box">
										<article class="image"><img src=""></article>
										<article class="excerpt small"><span class="category">Category 1</span><a href="">The title</a></article>
									</article>
								</div>
<div class="grid_unit">
									<article class="box">
										<article class="image"><img src=""></article>
										<article class="excerpt small"><span class="category">Category 2</span><a href="">The title</a></article>
									</article>
								</div>

							</section>
						</div>
					</div>
				</li>
</ul>
</nav>