Why do you need SOAP message ?
Are you trying to program a web service client to send SOAP message to a service provider?
Here you are doing this which is not good design.
Java Object (JAXB) -> String (XML) -> Java Object -> Java Object (SOAP) -> String (SOAP).
my query is how can we convert this plain xml into a SOAP req
To build SOAP message, you do not need to unmarshall object to string.
There are web service APIs like JAX-WS which has support for JAXB and SOAP API. Through this you can develop a WS client program which builds SOAP message for you.
Regards, Sujeeth Pakala
SCJP, SCWCD, SCBCD, ICAD, XML Master Basic, Certified SOA Professional