Hello Tanmeya,
If I have understood you correctly, you already have SSO for SAPGUI working so you've already done all the AD and SAP work (Service User/UPN/SPN/Parameters/Keytab etc ....)
You just want to extend your SSO now to use SPNEGO for your WebDynpro.
If my understanding is correct then the SPNEGO wouldn't be that complicated (only validated on 702 SP14).
1. Add a new SPN to your existing AD service user:
eg: HTTP/<hostname>.<FQDN>
2. run tx SPNEGO on the ABAP side and fill in the UPN and password
3. Activate session management with tx: SICF_SESSIONS
Hope that helps if I have correctly understood your requirement.
KR,
Amerjit