In the webservice call action select the custom option on the request header tab. Then select the insert template option WS Security User Name Token and your header will look like:
<wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
<wsse:UsernameToken>
<wsse:Username>{1}</wsse:Username>
<wsse:Password>{2}</wsse:Password>
</wsse:UsernameToken>
</wsse:Security>
Almost what you need add the additional tag and the namespace declaration in the template and map the variables in the parameters section and you should get the required structure in your webservice header when calling the webservice.
If the template isn't completely what you need then just write the header manually and add the needed parameters and you have the same results.