Hi Sri,
You can use whatever middleware you want to use. You should be able to still use the SAP add-on (PA_SE_IN) for the integration configuration on the SAP side, but you'll need to build the middleware mappings and configuration manually if you don't use Dell Boomi AtomSphere or SAP HANA Cloud Integration (HCI). This is because SAP provides packaged integration content for both Dell Boomi AtomSphere and SAP HCI. Both of those middleware platforms are included in the Employee Central subscription.
Best regards,
Luke