There are two ways to "nest" or otherwise combine IVRs.
The first way is to use the sub-menu system list, as mentioned. Simply create two or more IVR menus as if they were independent menus, with each one having a unique name. Then, from the main IVR, create an entry option with an action of menu-sub and a param containing the name of the child IVR.
<entry digits="1" action="menu-sub" param="child_ivr"/>
The advantage to creating your menus this way is that you gain the ability to use the menu-back action to allow callers to get to the previous IVR menu, useful if you might have multiple parents calling the same child menu.
The other way to use sub-menus is to assign each IVR a unique extension number and simply transfer the caller from one extension to another, in order to get to and from the parent and child menus. In this way, you can always guarantee that you can get from one specific IVR to another and back again, regardless of how an IVR was invoked.