You cannot combine interactive forms together. If you flatten them (take away the interactive nature so that they are not fillable) then you can combine them. You would the Assembler service of LiveCycle to do this (both the flattening and the assembling).
I thought you were doing this after the forms are rendered. If this is a design time experience then copy and paste should work fine. Note that you will only be able to have one schema so you will have to combine all schemas together.
how to use Assembler for flattening forms? As I understood the documents should be flatten before assembling and this will be not done by the assemler itself?
Yes ..I believe it was introduced in version 8.1. You can use a test application that ships with the product that will allow you to test your DDX file. If you look under the adobe/configurationManager/deply folder there is a file called adobe-assembler-ivs.ear. Drop this into your app server deploy folder then you can access the sample bu going to this URL: