Readme file : http://download.oracle.com/otn/nt/middleware/12c/1221/wls1221_D-PCT-README.txt
" ... The Domain to Partition Conversion Tool (D-PCT) provides the ability to migrate an existing WebLogic Server 10.3.6, 12.1.2, 12.1.3 or 12.2.1 domain to a partition in a WebLogic Server 12.2.1 domain. This tool consists of two components: an export tool, which is used on the WebLogic Server installation that hosts the source domain; and an import tool, which is used on the target WebLogic Server 12.2.1 installation. You can use this tool to create and configure partition, resource groups, and resource group templates. By default, this tool moves all applications, libraries, and resources to the new partition. Optionally, it also provides a mechanism for selecting individual applications, libraries, and resources. ... "
To use the D-PCT Tool you need to download (from the previous link) and install patch 22644507 on the target WebLogic Server 12.2.1 installation