The best option would be to import only issues you would like to use for your reports. You can use JQL query in import options to exclude any issues you would like to omit in your reports.
While there is an option to use MDX calculations to split internal and external issues using issue calculated members with function MATCHES on issue member name, they might give you unnecessary complexity for reports you would like to avoid.
You can always consider defining a new JavaScript calculated custom field during data import if you would like to split Internal and external issues in reports and analyze them side by side.
Daina / support@eazybi.com