Question:
Does XenMobile 10.x support the addition of a shared Mailbox in Secure Mail For both iOS and Android ?

Answer:
Currently this is not supported on Secure Mail

ActiveSync protocol officially doesn't support shared access/delegation. Hence, Secure Mail (both iOS and Android) also doesn’t claim support for this scenario. Below are few related links from Microsoft:
 
https://technet.microsoft.com/en-us/library/aa998357(v=exchg.150).aspx
 
https://blogs.msdn.microsoft.com/webdav_101/2015/11/10/delegate-access-with-exchange-server-activesync-eas/