Hi, I'm not sure I understand exactly how your process starts. Does it start from a K2 SmartForm? If you can see the Workflow start rules on a K2 SmartForm then opening up the rule will expose datafields configured in the workflow, you can pass in the Comment into any of the Workflow datafields. 

What is important is that you can find a 'holder' for the comments, if you can save the comments into a smartobject then you can load the value of that smartobject into a datafield on the workflow and pass that datafield into any other form or smartobject you might use.

The workflow starts when a user clicks a button on a document set SmartForm. I could not see any "Workflow start rules" when I pulled up the Rule Designer.  All I saw was Workflow Events -> After the Workflow action was submitted.  Anyway, I was able to get it to work by creating a column on the SharePoint document set, to hold the originator comments.  In the workflow, I have an activity that takes the orignator comments in the document set item that was created, and adds it to the comments table (or wherever it's stored in the K2 schema).