frozenColumns attribute is there for rich:extendedDataTable.
Due to the complex mark-up involved in the rich:extendedDataTable component, it does not support the use of the rich:collapsibleSubTable component. The rich:collapsibleSubTable component is only available with the rich:dataTable component.
Similarly, complex row and column spanning using the breakRowBefore, colSpan, and rowSpan attributes is also not available with the rich:extendedDataTable component.
I am also searching for a solution, but i am thinking to implement with a bit different design, as @Vasil mentioned we can use row count and pagination.
The below code uses row count and pagination
<div style="overflow:auto; height:500px; width:100%;" id="ere">
<p align="center">
<rich:dataScroller for="table" maxPages="5" />
</p>
<rich:dataTable width="1200" var="record" id="table" border="0" cellpadding="0" cellspacing="0" styleClass="tablemargin tableDetails"
value="#{customerController.topCustomer}" rows="2">
<f:facet name="header">
<rich:columnGroup>
<rich:column styleClass="width50">
</rich:column>
......
.......
<rich:column styleClass="width70">
.......
</rich:column>
</rich:columnGroup>
</f:facet>
<rich:collapsibleSubTable var="item" value="#{record.items}" rowKeyVar="rowKey" >
..........
.........
</rich:collapsibleSubTable>
</rich:dataTable>
<p align="center">
<rich:dataScroller for="table" maxPages="5" />
</p>
</div>