Unfortunately, it's not possible to simply shuffle your entire NAS drive library within the StreamMagic app.
The UPnP system is based around a play queue, so you must first add all the tracks/albums you want to play to the play queue and then select shuffle mode from within the queue. The order of the tracks in the play queue will then be shuffled.
Please note that most UPnP servers place a limit on the number of tracks that can be added to the play queue. While this varies from server to server, it's typically around 2000 tracks.
This is the process in which UPnP works and the vast majority of UPnP control point apps will work in a similar way.
It's also worth noting that some media servers may offer additional functionality, like the Jukebox function in Asset UPnP, for example.
In theory, this does effectively allow you to randomly play content picked directly from the server. Features like this may, however, require a subscription or payment and may not be supported by all Network Players or control point apps.