We currently have Exchange 2010 with HC9.x.
CAS
HUB
MBX
 
All on separate servers.
 
We are wanting to add 2013 into the mix.  I understand that HC will need to work some magic on their end, but I'm wondering if anyone can point me to either some documentation on how they added 2013 into their existing AD Domain, or if anyone can share how they brought 2013 into the mix, etc...

If you want to add Exchange 2013 in same AD forest where Exchange 2010 is for migration purpose then yes you can run in mix mode. The only requirement from Microsoft end is to ensure that Service Pack 3 is install on Exchange 2010 Server.