How to access Sublist using Workflows ?

Accessing Sublist using workflows is pretty much possible using Suiteflow. 

As per the draft version from release notes of 2019.2 is :
 you can set the following workflow actions to execute on supported sublists:

■ Return User Error
■ Set Field Value
■ Set Field Display Type

This enhancement increases the functionality on sublists and allows for greater customization of workflows. For an overview of SuiteFlow actions, including action properties, triggers, and conditions, see the help topic Workflow Actions.
To set an action to execute on a sublist, in a workflow, select the state that you want to add the action to. Click the New Action icon in the context panel. In the New Action window, click an action. In the Triggering Client Fields section of the Workflow Action window, choose Sublist, and complete the parameters and conditions section of the action. When you are done entering the action details, click Save. For more information, see the help topic Creating an Action.

For more details, please download the release notes from SuiteAnswers.