Hi Katherine,
Unfortunately Designer creates an XFA PDF form, which is not inherently compatible with AcroPDF files. (Their internal structure is different.)
You could try one or both of the following things and see if they work - I haven't the opportunity to test these just now myself, but this is what I would try..
Option 1 - When saving the table in Designer, select save as type option of "Acrobat 6 compatible static PDF" option and try if this can be merged into the rest of the file.
Option 2 - After completing all changes, see if you can set up a batch process in Acrobat professional to print the Design files back to AcroPDF format, and then merge into the bigger file.
I hope this helps,
Sanna