You can connect them as long as you have the correct cable.  your old CX600 DAE2's will use a HSSDC connector while the CX340 will use a HSSDC2 connector.  So you need a HSSDC-HSSDC2 cable.  Another thing to remember is that if your CX340 is running all buses at 4G,  attaching the DAE will not work untill you run the bus reset wizard which will throttle everything down to 2G.   hopefully you hvae an empty bus and can place those DAE2's onto it and not hurt the rest of your system.   That reset wizard is a disruptive process and will require all hosts to be powered off so nothing is accessing the lun's.  Your best bet is to place the DAE2 chassis on Bus1 and keep Bus0 as it is so it can stay at 4G incase you add future DAE3p enclosures
We currently have a retired CX600 with a dozen DAE2 enclosures and a CX3-40 with just the first tray.  Is it possible to connect the DAE2s to the DAE3 on the CX3-40 so that we can utilize the storage that we already have?  I am not concerned about slower performance as it is used for offsite storage.
Connect DAE2(CX600) to DAE3(CX3-40)