PynamoDB: how to update an attribute in a ListAttribute of MapAttribute

Question

I am learning how to use PynamoDB to work with AWS DynamoDB in my Python code. Here is what I have defined so far:

class OfficeEmployeeMap(MapAttribute):
    office_employee_id = NumberAttribute(hash_key=True)
    office_employee_direct_ids = UnicodeSetAttribute()

class Office(Model):
    class Meta:
        table_name = 'OfficeModel'
    office_id = NumberAttribute(hash_key=True)
    employees = ListAttribute(of=OfficeEmployeeMap)

Office.create_table(read_capacity_units=1, write_capacity_units=1) 

Here is how I appended to the list of map attributes (employees):

emp1 = OfficeEmployeeMap(
    office_employee_id=111,
    office_employee_direct_ids={"222"}
)

Office(
    office_id=1
).update(
    actions=[
        Office.employees.set((Office.employees|[]).append([emp1]))
    ]
)

And here is what I am seeing in the newly created DynamoDB table OfficeModel:

Now I need to add another value, like 333, to the set office_employee_direct_ids where office_employee_id == 111, how can I do it with PynamoDB? Thanks for your help!