Unfortunately, you cannot do that, and here is why.
Although the article talks specifically about connecting to a database, a similar thing occurs when trying to use PHPMyAdmin to connect to a database outside of InfinityFree.
Our phpMyAdmin is linked to our database servers. You cannot use our phpMyAdmin installation to access databases hosted elsewhere.
If you have a database somewhere and you want to access it with phpMyAdmin, you can also install phpMyAdmin on your own website and use that to access to external database.
Please do not that the external database must support external connections and accept them from our network.