Merge three sites

I currently have three sites that share a users database. The sites are set up in multisite. We have recently decided that we would like to share content in addition to sharing users. After doing some research, I think Domain Access will do the job. Now I'm faced with the challenge of getting content from all three sites combined into one database... Does anyone have any experience or suggestions on how to approach this? I'm thinking of either using the node_import module or going with a custom script that will increment the node IDs across the site (could get messy...).