Write the same expression separately for selecting each individual column to convert to array.
Create a variable of type System.Collections.Generic.IEnumerable<system.string>
Write an assign to append each array to the Collection e.g. Collection.Append(Array1)
This will provide you a list of string arrays e.g. {[A,B,C],[D,E,F],[G,H,I].
Alternatively if you want to have a single list of individual elements e.g. {A, B, C, D, E, F, G, H, I} then each time you create an individual array from a column use a ForEach Loop through that array and append it to the collection one by one (make sure to change the collection type to string rather than string).
There is no doubt a more elegant way to do this but it will get you there.