RECODE in STATA

recode var names (1 2 = 1 low)  ///
                 (3   = 2 medium)  ///
                 (4 5 = 3 high)  ///
                 (nonmissing = 9 "something else")  ///
                 (missing = .)  ///
               , gen(Rx Ry) label(Cat3)

Use mvencode and mvdecode to deal with missing values. 
For example to convert 99 into sysmis
in the variable mstatus,

mvdecode mstatus, mv(99)

For the opposite effect,
mvencode mstatus, mv(99)