I have ST2 setup so that I can do 'sublime file.txt' and it will open in a ST2 window. But how can I make it open in a new tab in the currently open window?

Try Sublime command line help

subl --help Sublime Text 2 Build 2217

Usage: subl [arguments] [files]         edit the given files
   or: subl [arguments] [directories]   open the given directories
   or: subl [arguments] -               edit stdin
Arguments:
–project <project>: Load the given project
–command <command>: Run the given command
-n or –new-window:  Open a new window
-a or –add:         Add folders to the current window
-w or –wait:        Wait for the files to be closed before returning
-b or –background:  Don’t activate the application
-s or –stay:        Keep the application activated after closing the file
-h or –help:        Show help (this message) and exit
-v or –version:     Show version and exit
–wait is implied if reading from stdin. Use –stay to not switch back
to the terminal when a file is closed (only relevant if waiting for a file).
Filenames may be given a :line or :line:column suffix to open at a specific
location.

After you have opened a new window the subsequent files should be added there as per the default behavior.