Help for Iterating through a list and Concatenating more values

I have created a table with a column of devices and a column that contrains lists all devices downstream of the device in the first column.  However I need to create a column to check if the items in the list in the second column are also in the first column and if so find the list of downstream devices for each of those devices and append it to the origonal list.

It should look a bit like this

Here is a screen shot of the data i am working with

Thank you for the help!