Pandoc inserts styles with names in english. LibreOffice (OpenOffice) default styles can not be renamed. Therefore, you need to modify the styles with the stylel you wish to use. If you start with a odt generated by pandoc, can write the needed styles (using the same names as Pandoc uses). To write the styles, you need to open the vertical bar at the right of LibreOffice and select the operation you wish to do.

The "default.opendocument" only have:

 -  $automatic-styles$
 -  $header-includes$
 -  $title$
 -  $author$
 -  $date$
 -  $include-before$
 -  $include-after$

Therefore, you need to examine wich automatic style applies to the fragment that needs that style adapted to the APA specification.

I include also an template for APA 6 reports and anothe one for the previous APA, with the hope that could give you resources.