Brian,
PDF Files made with Acrobat are based on postscript.
PDF Files made with LiveCycle Designer are based on XML and are essentially wrapped in a PDF. Once modified in LiveCycle Designer, you can no longer edit the file directly in Acrobat.
They do not share the same base file type and can not be combined into a single PDF using Acrobat version 7.
One of the new features of Acrobat version 8 - is called packages. Packages allow you to combine files of different types (acrobat and designer) as well as documents with different security settings (also not previously allowed). Each file in the package maintains it's independent identity and settings, wrapped in a new pdf file.
Recommended solution: Upgrade to Acrobat 8 Professional, get the new version of designer, and the ability to use packages.
Hope this helps.
Angie Okamoto