Merging Sites

Hello,

I have two sites that I’d like to merge into one. Both running on their own domains, not subdomains.

What I’ve done so far is move Domain A to a subdomain ready for testing, I’ve exported from Domain B and have imported to Domain A with little success. It’s created users but not categories or posts. Is there a plugin anyone can suggest that will give me greater control and offer debug options?

Also, if I’m using a heavily integrated plugin such as Powerpress, will I need to install the plugin on Domain A before importing from Domain B? Should the import/export tool be intelligent enough to know there was a plugin on Domain B and create the settings on Domain A?

Thanks