Pulling array field values in a dropdown

Hi Experts 

I have two tables and i have defined link between them, For ex. Main Table and subcategory table.

In subcategory table i have defind one array field(assignments) where i am adding assignments for the subcategory.

Now in main table i have defined one varchar field and via link i want to pull all the assignment group values as dropdown or fill button based on the select subcategory.

For Ex:- We have a subcategory as "Error Message" and in subcategory table assignment field i have defined three assignment values like "Support Team 1", "Support Team 2" and "Support Team 3",. Now whenever i select subcategory in main record i want these three values in assignment field. 

In my experiane "Everything is possible in service manager" we just need to know the correct way. I am using latest verion of service manager(9.41), not sure whether PD can help here.

Any clues on above requirement ?