Importing content won't affect existing permissions, assuming members are placed in the existing user groups in XF and new groups aren't created during the import.
Any new nodes importer will have to be set up, if necessary.
Also, you won't be able to do a 'top up' import later, if that's what you're planning.
You can import more content, but you can't import again from the same forum and only import new members, posts, etc.