Re: Can I export/import users between projects? ...
It is possible within Site Admin to copy users from one project to another. On the Site Projects tab, select the destination project, click on the Project Users tab for that project, pull down the Add button and select Copy From Another Project. That will give you another pane on the right with a tree view of all the domains/projects. Find the project that has the users you want. Check mark the project itself to get all users, or expand the node and check mark individual users in the project. Click the Add Selected Users To Project button.
I'm not sure what it does about the user's permissions in the new project. They get at least Viewer permissions. I'm not sure if they get the same group assignments they had in the source project.
It would be possible to do this using the OTA API.
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)