I don't remember off-hand how often Asset Extensions get updated, but, for the second part, of how to connect to the other source, the easiest option is to set up a linked server connection from the VCM SQL Server to the CMDB one (see Linked Servers in SQL Server documentation). Then you can create Asset Extensions dynamic queries that reference the linked server as the source for the information.
A second option would be to set up a SQL Server Integration Services package to periodically pull data into local tables on the VCM SQL Server, and query the AM data from there.