Soundiiz is able to import multiple playlist file formats and also plain text directly.
For the XML format there's a schema to have to be sure to working well with Soundiiz system.
When importing a playlist as XML file
<title>Alle Skuffer Over Tid - Single Edit</title>
<artist>The Minds Of 99</artist>
<album>Alle Skuffer Over Tid</album>
Only the track "title" field is required, leave empty other fields if you don't have the value for it.
This XML format is also the one exported by Soundiiz for your playlist. So you are able to export your Playlists as XML and then, later, import them back to Soundiiz in this same format.