decryption problem with MD5

[zeytunak]

Its magic , I want to reduce possible string but increased rapidly 6^16=2821109907456 .

It is used as login and in first time that set by user then hashed value saved and then used to check valid user login.

this encryption is made from "md5 algo" + "simple adding algo" and adding is not part of md5 and programmer used adding after using md5 .
( instead of using non-standard md5 , maybe programmer dont know about non-standard md5)
I think this is good idea becuase if programmer only use standard md5 for fixed string length ( 16 char in our example ) then its easily breaked. all possible 16 char string maked and hashed with md5 and search between them .
many site provide standard md5 decryptor in above manner .