You can do :
case when A = 'B' then 'C' else ( case when A = 'D' then 'E' else ( case when A = 'F' then 'G' else 'NA' end ) end ) end
or in a formula in a Prepare recipe :
if(A=='B','C', if(A=='D','E', if(A=='F','G', 'NA') ) )