In the PAC software, the Logibuilder allows you to add previously-defined HDL code modules to a Platform Manager design by using the Import Sub Module command under the File menu. Only one HDL sub-module can be imported. The last imported one will overwrite the previously imported sub-module.
Please refer to the PAC-Designer software Manuall. The section "Importing HDL modules to a Platform Manager Design" will provide you with more details.