Add a column to an Embedded Table, as a transformation. Lookup Previous Row.

Here is some code that should do what you need.

let
    nested_5493 = 
    #table(
        type table [OrderId=nullable number, OrderDate=nullable date, CustomerID=nullable number, Status=nullable text, Index=nullable number],
        {
            {496977, #date(2025,3,3), 5493, "Shipped", 0},
            {498804, #date(2025,3,10), 5493, "Shipped", 1},
            {514401, #date(2025,5,4), 5493, "Shipped", 2}
        }
    ),
    nested_5626 = 
    #table(
        type table [OrderId=nullable number, OrderDate=nullable date, CustomerID=nullable number, Status=nullable text, Index=nullable number],
        {
            {496988, #date(2025,2,3), 5626, "Shipped", 0},
            {498805, #date(2025,3,12), 5626, "Pending", 1},
            {514402, #date(2025,5,6), 5626, "Shipped", 2}
        }
    ),
    dim_table = 
    #table(
        type table [CustomerID=nullable number, Email=nullable text, dbo_Orders=table],
        {
            {5493, "[email protected]", nested_5493},
            {5626, "[email protected]", nested_5626}
        }
    ),
    add_nested_column = 
    Table.TransformColumns(
        dim_table, 
        {
            {"dbo_Orders", each Table.AddColumn(_, "PreviousOrder", (r)=> try [OrderDate]{r[Index] - 1} otherwise null, type date), type table}
        }
    )    
in
    add_nested_column

In the sample table I created...

I added a column to the nested tables that shows the previous OrderDate.