In the OOTB modules there are predefined phases to which an approval plan can be attached. If these workflows are extended by further phases, no approval plan can be attached to the new phases. This is also not possible for existing phases that do not have an approval plan in the OOTB. Only in completely new modules, which are built in SMAX, it is possible to set an "Enable approval plan" flag at each phase.
This possibility should be possible at every phase in every module.