Ok, then you're going to need use the Content Builder to modify the existing pipe fittings that you want to be included as an AutoLayout option.
Perhaps the easiest way to explain how to do this is to go to http://inside-the-system.typepad.com/my_weblog/2008/02/dusting-off-the.html and watch the video titled "Adding Eccentric Round to Rectangular Transition" Even though the video deals with changing duct transitions to be eccentric, the process is nearly identical for what you're wanting to do.
Just follow the same clicks as he does to open the Content Builder (note: if you don't have the CAD Manager pulldown, it's under Window > Pulldowns > CAD Manager).
Then browse to the pipe fitting you want to modify (you'll need to change the Part Domain pulldown at the very top to Pipe), and click the Modify button like he does.
This should bring open the fitting along with an additional sidebar on the left side of your screen.
Like in the video, click the Options button at the top right of the sidebar, but then check the AutoLayout Flag box and click OK. (If the AutoLayout Flag is already checked, then chances are the part you want is already in the AutoLayout list, it's going by a different name.)
Be sure to click the Save Part Family button at the top left of the sidebar, and close out.
For good measure, you may as well go ahead and regenerate your part catalogs (CAD Manager > Content Editing > Regenerate Catalog).
Then check your AutoLayout options again and with any luck, your desired part should now be in the list.
Unfortunately, I think you're going to have to do this for each fitting you want add, as I don't think there's an easy way to change multiple fittings at once.
Also, there may be a good reason why Autodesk didn't include the part as an AutoLayout option, as sometimes the parts don't behave the way as desired in AutoLayout mode.
Good luck,
-Brad