Send additional parameter to Ajax event listener

You could set an attribute in your data table and read it in your select listener. To do so, use <f:attribute name="..." value="..."/>. From the documentation:

Constraints

Must be nested inside a UIComponent custom action.

Description

Locate the closest parent UIComponent custom action instance (...). If the associated component already has a component attribute with that name, take no action. Otherwise, call the isLiteralText() method on the argument value. If it returns true, store the value in the component’s attribute Map under the name derived above. If it returns false, store the ValueExpression in the component’s ValueExpression Map under the name derived above.

So, taking the attribute you tried to set in your comment, you should use it like:

XHTML:

<p:dataTable value="#{aObj.objList}" var="item" .... selectionMode="single">

  <f:attribute name="test" value="abc" />
  <p:ajax event="rowSelect" listener="#{aObj.viewItem}" />

  ...
</p:dataTable>

Listener:

public void viewItem(SelectEvent event) {
  String test = (String) event.getComponent().getAttributes().get("test");
  // ...
}

When you want to add request parameters, which are specific to the ajax tag, you can do at the onstart method:

<p:ajax onstart="cfg.ext.params.push({name: 'name', value: 'value'});"/>

Usually, you should solve parameters using the model.