You can use a filter tool and branch off null values into another stream, then perform a join to the values you need to look up in another source, and then finally union that data back together.
And for replacing the #N/A error, the best way is to use IFERROR with VLOOKUP. IFERROR is a function which can help you to evaluate a value and if that value is an error it will return a custom value instead of that error.
While you could achieve what you are looking for with the join multiple tool, I believe there is an easier way.
In the example attached, I am first merging the list of active and completed cases, then performing a full join (VLOOKUP) using the join tool + the union tool.