You can combine table as below:
Table(
Distinct(
Filter(Positions, Status.Value <> "Closed without Hiring" && Status.Value <> "Hired"),
Title
),
{Value:"All Position"}
)
If in code above with Value doesn't work, then try {Result:"All Position"} or {Title:"All Position"}.
This will put "All Position" at the end of your Combobox.
Assuming you are using this ComboBox to filter gallery , then you will have to modify the formula for Item Property in your Gallery
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