Resolution:
Following are the two methods:

1] Create new xml file for new category type and put it into directory - \TIBCO\portal\CategoryTypes,  or modify the existing defaults.xml file in the directory - \TIBCO\portal\CategoryTypes. A sample defaults.xml file is attached

Following section is added in defaults.xml, that adds new category type "Test".

&ltcategory.template type.name = "tibco.test.html" type.ref = "tibco.root" type.label = "Test" type.order = "2">
&lttransformation>
&ltsource.http url = "http://" scrubURLs = "true" httpCacheDuration = "10000" name = "source1" cachePolicy = "auto">
&ltparser.unstructured name = "headline" dataStartsAfter = "<body[^>]*>" dataStopsBefore = "</body>" includeLineFeed = "true"/>
</source.http>
&ltoutput name = "portal" templateFileName = "Templates/Content/PreformattedHtml/ContentTemplate.htm"/>
</transformation>
</category.template>    
&ltcategory.template type.name = "tibco.passthrough.html" type.ref = "tibco.root" type.label = "Passthrough HTML" type.order = "2">
&lttransformation>
&ltsource.http url = "http://" scrubURLs = "true" httpCacheDuration = "10000" name = "source1" cachePolicy = "auto">
&ltparser.unstructured name = "headline" dataStartsAfter = "<body[^>]*>" dataStopsBefore = "</body>" includeLineFeed = "true"/>
</source.http>
&ltoutput name = "portal" templateFileName = "Templates/Content/PreformattedHtml/ContentTemplate.htm"/>
</transformation>
</category.template>


2] Refer PB API guide -> Chap 10 -> Pg 130.

For both the methods, web server\servlet engine restart is essential after the change.