Depend upon the position where we would like to insert your custom block you can insert your custom block.
Below is xml to insert custom block into product page
<catalog_product_view> <reference name="product.info.extrahint"> <block type="tag/product_list" name="product_tag_list" before="-" template="tag/list.phtml"/> </reference> <reference name="product.info"> <block type="catalog/product_list_related" before="-" name="catalog.product.related" as="related_products" template="catalog/product/list/related.phtml" > <action method="addToParentGroup"><group>detailed_info</group></action> </block> </reference> </catalog_product_view>