Exporting to most account packages via a direct interface (not a text file import) has the option to create new clients and suppliers as part of the export. This is only allowed if this is the only reason why a batch cannot be posted. Any other validation failures, such as analysis code or tax code failures must be resolved first. Then, once the batch is re-posted the option below will appear:
After clicking 'Post batch' and going through the usual process, for any new accounts you will need to click the button to create missing accounts, then click again on 'Post batch'.