Problem Definition

At this moment, it is not possible to update the "Fix Version" with a post function. 

Suggested Solution

Add a post-function that updates the value of "Versions" type of field.

Why this is important

Allow more flexible workflow design, reaching a broader range of businesses needs.

Workaround 

Admins can use add-ons to achieve this goal. A few names are: Jira Misc Workflow Extensions, Automation Lite for Jira and Automation for Jira.