Wow that's a blast from the past, and thanks for replying Kernel99. Since I posted the question, I have upgraded TL and found the solution to my question.
Yes it's true a trading partner can have their own map. But, the reason I was trying to condition the sender id was due to a majority of our trading partner maps were pretty much identical. So, I decided to consolidate these maps based upon the version and add any trading partner specifics to the consolidated map.
In order to condition the sender id, I performed the follwoing steps:
1. Create a workfield.
2. In the "ST" segment map the keyword <Partner Internal Id> to the partner workfield.
3. After this mapping you are able to perform conditional mapping on any other segment.
If you have any more questions, please feel free to contact me again.