The first step is to identify the code you already have, a program called IRTool will do this for you.
The second step is generating all codes in that set, the number of codes in a set depends upon the protocol of the codeset, but most often there are 255 codes. A program called Makehex will do this for you.
The third step is to test all the codes into your TV.
Both these programs are available here on RC.
Post the code you have and we'll give you some pointers.
Mark