Hello Marius,
I don't know the ECC version you have, what you can try is to use transaction SPROXY and the magic wand button to create service call:
Option Service Consumer, click on Cont.:
Use the external service definition, click on continue:
Your external definition comes from an URL, select URL option and click on continue, enter the URL of the WSDL and then create the proxy:
After that, you can use the transaction SOAMANAGER to create the logical port. The logical port will contain port and binding to the webservice.
For more information see SAP help file:
7.1
http://help.sap.com/saphelp_nwpi71/helpdata/en/9e/c7a3591dc74a679bbc9716354e42af/content.htm
7.11
http://help.sap.com/saphelp_nwpi711/helpdata/en/69/8a1e9553dc4baba6026a3db510cadb/content.htm
I wouldn't advise use the use of RFC<->PI<->SOAP as Best Efforts scenario can be resource intensive. PI still best at asynchronous communications. In your case a Point-to-point communication using consumer proxy is best.
Hope it helps
Regards
Yan Wong