Hi Catherine,
There could be some challenges there, but you have a few possibilities. Please bear in mind that I am not an HCM expert so I can only give some broad ideas.
1. Does your system support IDOC or XI? I've heard of scenarios where connections were made from those systems.
2. I've been on projects where SAP HCM was not the leading system. In this case we got the HR folks to prepare either a flat file or a database table holding the new / changed HR entries and started processing from there.
3. If it's possible to expose the tables, you could also use VDS to create an LDAP representation of the HR data, this could be configured read only and you can put some pretty good security in place to protect the data. Additionally VDS can be configured to only expose certain attributes.
These are just a few ideas, also of course you could start a project to migrate to a later version of HCM or to Success Factors, both of which are supported by IDM 8.
Hope this helps!
Cheers,
Matt