Yes it's possible. Using the example on the bootstrap site as a basis, add a submenu to the existing submenu.
Like this:
<ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu">
<li><a tabindex="-1" href="#">Action</a></li>
<li><a tabindex="-1" href="#">Another action</a></li>
<li><a tabindex="-1" href="#">Something else here</a></li>
<li class="divider"></li>
<li class="dropdown-submenu">
<a tabindex="-1" href="#">More options</a>
<ul class="dropdown-menu">
<li><a tabindex="-1" href="#">Second level link</a></li>
<li><a tabindex="-1" href="#">Second level link</a></li>
<li><a tabindex="-1" href="#">Second level link</a></li>
<li><a tabindex="-1" href="#">Second level link</a></li>
<li><a tabindex="-1" href="#">Second level link</a></li>
<li class="dropdown-submenu">
<a tabindex="-1" href="#">Even More options</a>
<ul class="dropdown-menu">
<li><a tabindex="-1" href="#">Third level link</a></li>
<li><a tabindex="-1" href="#">Third level link</a></li>
<li><a tabindex="-1" href="#">Third level link</a></li>
<li><a tabindex="-1" href="#">Third level link</a></li>
<li><a tabindex="-1" href="#">Third level link</a></li>
</ul>
</li>
</ul>
</li>
</ul>