Adding reference from other process to BPMN diagram

I am modeling a process with BPMN diagram (it was initially created with a non-BPMN model) and I would like to know what is the best way to input a step from another process to a specific step in the BPMN diagram.
I thought of using a call activity or a message to achieve the above, but I would like to know the best practice for this.

Thanks in advance,

You are absolutely correct. From the BPMN point of view a Call Activity is a reference to an external BPMN process.