You can build a World Adventures basement under a house with a foundation that is accessible.  What you need to do is delete at least 5 squares of the foundation, place a set of small stairs from the foundation to the bare ground, then go to the basement and place the stairs using the remaining ground squares, but make sure you leave one empty ground square for the transition between the two sets of stairs.  You can place a floor tile on the empty ground square. Yours sims will pause at the transitional ground square, but it will work.  If you can't get your sims to go to the basement, try clicking on the basement stairs and then "use stairs."  I have some screenshots for a little more clarity: