I worked with Manufacturing companies and none of the companies were using any ERP for HR process. So opportunities for me is limited to grow and wants to have an edge over other HR people. I haven't come across any company which is looking for core HR person and will give training on the ERP as I am already experienced one and obviously expect me to come with experience in the field.
Its been great to know that SAP- HR Functional has wide options but few companies in practice. However, You have to hit the CMM level companies to pursue any such prospects. As you have a former experience in Human Resources it should be easier to get through. Certainly, SAP has a certification course and there are many oppurtunities available. Good Luck to you!!