Hi Dileep,
In case you need to go ahead with custom solution below is the approach you need to follow.
1. Activate the Template and webservice in Ariba end
2. You need to generate the RFX data in CSV Files as per template of step1 and encode the CSV Files using base64 format and has to post to the Ariba Realm using webservice
3. Once receives the bidding responses and awarding is done.
4. you need to pull these response to have the corresponding PO/CTR creation in ECC. Again a webservice is available and you need to pass the sourcing project id to fetch this info.
Also there is a standard one released for tactical sorucing solutions called spotquote which has become available with CI-3 Adapter release functionality.
HTH
Rajesh