Sketching Sub-process in RRC

I find the best way to sketch the sub process is to create a separate process artifact and link this new artifact to the sub process element on the original process sketch.

FYI here is an overview of the available Business Process elements
https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_define_biz_process.html&scope=null

A subprocess can be broken down into a finer level of detail (as a process) through a set of subactivities.

as well as this guidance on adding elements to a sketch might help

https://jazz.net/help-dev/clm/index.jsp?re=1&topic=/com.ibm.rational.rrm.help.doc/topics/t_define_biz_process.html&scope=null

After adding a subprocess element to the editor, double-click the subprocess to link it to a new or existing simple process diagram.