You can create drop down navigation pages by using the following steps:
- Click on the + icon in the template page navigation
 - Click the Convert to portfolio button
 - Click on the drop down arrow in the Portfolio page navigation
 - Click the Add a new page here button to add a new template page or the Add an existing asset button to add an existing asset
 
Repeat steps 3-4 as many times as required to add multiple pages to this Portfolio.
The main builder interface restricts to a single level of nesting, meaning you will be unable to add a Portfolio inside a Portfolio with the instructions above.
If you would like something a little more complex then is currently possible with a workaround.
What you would need to do is:
- Create your Portfolio(1) with a single level of nesting (see steps 1-4 above)
 - Save it
 - Create a new Portfolio(2) (see steps 1-4 above) and add Portfolio(1) by selecting the Add an existing asset option during step 4
 - Save it
 - Create a new Portfolio(3) (see steps 1-4 above) and add Portfolio(2) by selecting the Add an existing asset option during step 4
 
The example above will result in having a Portfolio inside a Portfolio inside a WorkBook, 3 levels. You can repeat the above for as many levels as you require.