Both systems (Genesys and ProCentre) are controlling the call on the same route points/extension numbers independent of each other. That means, ProCentre is playing music to the call, whilst Genesys has Queue'd the call and is looking for a Target. When Genesys finds the Target, it removes the call to the Agent's DN/Ext, effectively removing it from the original route point/extension - away from ProCentre.
You are right that you might want to consider 2 steps for this to be effective. The first step (the same DN, configured as a Genesys Route Point and a ProCentre ACD Ext - e.g. 1000) will ensure that Genesys is actually up and running and needs to have a simple Genesys Routing Strategy to move the call to the second step. The second step (another DN, configured as a Genesys Route Point/Queue and also configured as a ProCentre Music Port - e.g. 1001) would ensure that ProCentre plays music (keep adding multiple 300-second intervals of Music for the Port, up to 3000...) via it's own strategy/vector (or whatever they call it in ProCentre-speak) whilst Genesys is Queueing the call on the same point, as it attempts to find a Target. As Genesys either finds a Target, Time's Out or has to Default Route the call, it is effectively released from the second DN (1001), meaning it will take back the call from the ProCentre Music Port and send it on to wherever you have defined in the Genesys Routing Strategy.
I expect a picture is worth a thousand words for this - unfortunately, you got the 1000 words...

I hope this helps?
Tony