You can certainly do this with Lag() but you might also be able to do this interactively in JMP (not sure which version gives this option, I'm using JMP16.1... there are also addins to do this).
Select the column you want to fill
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Right click on top of the values and select Fill / Replace Missing with Previous value
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Edit:
With Lag one way to do this is to create new column and use formula like:
If(Row() == 1, val = :Status);
If(:Status == "",
val,
val = :Status
);
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-Jarmo