A,B,C,D
1,1990,I001,2473264
2,1991,I002,2473265
3,1992,I004,2473266
4,1993,6050,912432
5,1994,6003,912433

A,B
I001,2.3 GHz
I002,2.3 GHz
I004,2.3 GHz
6050,1.8 GHz
6003,850 MHz

A,B,C,D,E
1,1990,2.3 GHz,I001,2473264
2,1991,2.3 GHz,I002,2473265
3,1992,2.3 GHz,I004,2473266
4,1993,1.8 GHz,6050,912432
5,1994,850 MHz,6003,912433

awk -F, 'NR==FNR{a[$1]=$2;next}a[$3]{print $0","a[$3]}' test2 test1

A,B,C,D
1,1990,I001,2473264
2,1991,I002,2473265
3,1992,I004,2473266
4,1993,6050,912432
5,1994,6003,912433
6,1995,6004,21234

1,1990,I001,2473264,2.3 GHz
2,1991,I002,2473265,2.3 GHz
3,1992,I004,2473266,2.3 GHz
4,1993,6050,912432,1.8 GHz
5,1994,6003,912433,850 MHz
6,1995,6004,21234,0

awk -F, 'NR==FNR{a[$1]=$2;next}{print (a[$3])?$0","a[$3]:"NA"}' SECONDfile FIRSTfile