Feature request: convert missing values

[dusadrian]

SPSS, Stata and SAS all have different ways of dealing with missing values. The ReadStat library is awesome, but still leaves this uncovered.

For instance, an SPSS variable can have these values:
1 'Yes'
0 'No'
-1 'Non-response'
and it is possible to define -1 as a missing value.

In Stata and SAS however, missing values are not numbers but letters, such as: .a or .b
Would it be possible to define (perhaps in the .json metadata file) some sort of transformation instructions?