Welcome to the forum. To add a new PC to the game startup, you'll need to delve into .2da editing. There's a tutorial for the whole process here
, as well as an entire Tutorials Sub-Forum
available to assist you.
All tutorial sub-forums are available in Holowan Labs, so you've got the majority of the resources you'll be needing right there.
Since you're new here, you may also want to stop by this thread
, and introduce yourself.
Be polite, check your spelling, and ignore the trolls for a peaceful stay on LucasForums.*
*Peace? Are you kidding? THIS. IS. LUCASFORUMS!