How do I optionally save a custom data output to an array? A script task custom

you could either:
a) have a gateway that controls the process model flow and configure it so if pv!newStaff.hireDate>today() go to "scriptTaskToWriteToArray" else goto "nextScriptTask/skipWriteToArray".
b) you can filter out the blanks of the pv in another script task using the reject function and re save into the array.

If you're trying to handle the whole pv!newStaff array at once, you can probably make use of the index and where functions, such as:
index(
pv!newStaff,
where(pv!newStaff.hireDate > today())
).firstName

Instead of using 'appended to' in the output condition, use append() in the expression and choose 'save as' for the condition. ex:

if(pv!newStaff.hireDate > today(),append(pv!YourArrayHere,pv!newStaff.firstName),pv!YourArrayHere) -> Save As -> pv!YourArrayHere.

Which will essentially either append a value to pv!YourArrayHere, or save the array back to itself (i.e. doing nothing).

You can also create an expression rule with following input parameters and definition.

Inputs : index(int), hireDates_DateArr (Date),firstNames_txtArr(Text)

Rule definition :
if(
ri!hireDates_DateArr[ri!index] > todate(
local(
now()
)
),
ri!firstNames_txtArr[ri!index],
null
)

Do apply on this rule when you have multiple hire dates and first names to filter.

=apply(rule!aboveRule,{enumerate(length(ri!hireDates_DateArr)+1)},ri!hireDates_DateArr,ri!firstNames_txtArr)

Note : todate(local(now())) gives exact today's date considering GMT time before 10 AM and after 5 PM.

Please add appropriate null checks for hireDates, and firstNames.

You can make a rule for removing a value from a particular index of an array(let's say rule!RemoveValue) with the following definition:

remove(ri!YourOutputVariable,ri!ValueToRemove)

After that, you can call the above rule in another rule like this:

apply(rule!RemoveValue,{wherecontains(null,ri!YourOutputVariable)},ri!YourOutputVariable)

Now, you can use this rule to filter null values from your array.