How to add data from one Drupal site to other? Help me solve this scenario,  

Assumptions:


Two users user1 and user2 are using their own copy of Drupal site - site1 and site2 on their respective machines.  
They use same set of contributed modules (possibly with different configuration of the modules).
Both users add about 300 pages each to the sites.


Now, it is decided that the organization will use the site of user2 and migrate it to the production server. The organization also wants the pages created by user1 to be added to the site created by user2. How can this be done without changing the configuration of the site?

I did use the Backup and Migrate module to migrate the site site2 of user2 to the production server. Now, I am having difficulty in adding the content of site1 created by user1 to the production site; because if I use Backup and Migrate module again it will overwrite the existing pages from the site. It will also change the settings of the modules stored in the database. It is because, the nodes (pages) on site2 and site1 will have nids starting from 1 and auto-incrementing in the nodes table. So, doing a backup and migrate procedure will replace nid=1 from site2 with nid=1 from site1.

Is there a way so that I could append nid from the old database to new?

What other options do I have?
You need to migrate the content of site 1 keeping the config of site 2. My recommendation is to use migrate module coz I prefer it. But you'll have to write code to use migrate module. 

There are other modules that may help you like node export, deploy. You can also write custom script but I have had most success with migrate module. You might need to justify the use of migrate in comparison to the content to be migrated, because you'll need to write migration classes but for lets say 300 nodes I'd still use migrate.

I wrote an answer sometime ago which talked about migration of content, users, categories etc. See if it helps you to choose as well - Updating content after major upgrade from 5 to 6 (changes to production site need to be reflected in dev environment)