In Video Single feeds, the <CollectionList> is used to add Music Videos to playlists on YouTube. The <CollectionList> is not used in Sound Recording feeds.
To include a video in an existing playlist, you can use the dedicated "YOUTUBE:PLAYLIST_ID" namespace inside the <CollectionId>, with a <CollectionType> of "FilmBundle".
<CollectionList>
<Collection>
<CollectionId>
<ProprietaryId Namespace="YOUTUBE:PLAYLIST_ID">PLONRDPtQh-FLMXFMM-SJHySwjpidVXmzw</ProprietaryId>
</CollectionId>
<CollectionType>FilmBundle</CollectionType>
<CollectionReference>X1</CollectionReference>
<!-- If title is provided, it will update the title of the existing playlist. -->
<Title>
<TitleText>My Updated Playlist Title</TitleText>
</Title>
<CollectionResourceReferenceList>
<CollectionResourceReference>
<!-- Adds the video at the start of the playlist. -->
<SequenceNumber>1</SequenceNumber>
<CollectionResourceReference>A1</CollectionResourceReference>
</CollectionResourceReference>
</CollectionResourceReferenceList>
</Collection>
</CollectionList>