When joining components with the Join Elements tool BIMpak needs a way of identifying the elements in the structural framing element to join to it. The way it determines this is by searching the family name of the elements for pre-defined values. This page of the BIMpak Settings contains the list of strings to look for. The family name only has to CONTAIN one of the values on this page to be included in a join.
The default list of values are: ADDON, CORBEL, PILASTER, LDGE, LEDGE, CORNICE, SOLID_ZONE_RECTANGULAR and SOLID_ZONE_TAPERED.