Phillipe,
I think the question really is about integrating SAP HR with the third party HR systems as EHSM requires the people data to either be in SAP HR (as mini-masters as a minimum) or the other approach is to have the people represented in SAP as business partners.
However with use of business partners the concept of an organisational structure and use of this for workflow is a challenge.
Thus the best option is to integrate the third party HR systems with SAP HR for EHSM to then utilise. Depending on the HR systems this should be entirely feasible (suggest you talk to SAP HR specialist to valdate).
Regards,
Michael