Is there an Excel "missing value" or "NA" symbol?

You're almost there. You can use NA() in your formulas to return that "error" value. It's very useful when charting because it's not charted (unlike 0).

You can leave cells empty and use (if I remember correctly) =isblank() to do the conditional. And it's worth noting that many numerical functions like =sum() and =product() will ignore empty cells, ala sum(x, na.rm=TRUE) in R.

But no, this is one of dozens of reasons why Excel is not a serious tool for statistical data. (Although it's not terrible for simple simulation and optimization problems.)