Assuming I understand you correctly....
Irrespective of manufacturer, if its a RAID box, not a JBOD, then no, you're not going to be able to plug in A.N.Other drive (whether Mac, Windows or Linux formatted) and see it come up as an independent drive.
All you're going to do is bugger up your data when the RAID subsystem decides to adopt the newly inserted drive.
Of course, if the synology system has some special software on it, then I'm willing to be corrected.... however even if that were the case, I still wouldn't trust it.
You may be better off just synching your data from A to B on a regular basis.