It is ablable to specify a process variable to calledElement of callActivity in BPMN specification.
However, I can't specify a process variable to calledElement with new process designer of Business Central.
Only to specify option of dropDownList.
So I edited bpmn file as follow,
<bpmn2:callActivity id="_F29CE5B8-1166-432E-BAE0-26C7FD064035" drools:independent="true" drools:waitForCompletion="true" name="Sub-process" calledElement="#

">

Then, I did git push the edited commit to Business Central Git repository.
But process designer's diagram show it "null".