You want the position number to be constantly added to a combo/list? $insong.pos returns the number of milliseconds into the currently playing file...not a very useful thing to add, as text, to a combo/list.
However, if that really is what you wish to do, you need to use a timer to constantly update the listbox with your $insong.pos.
.timerMP3 0 1 did -ra MP3Player 1 $!insong.pos
This breaks down as follows:
.timerMP3 is the named timer (so you can stop it easily whenever you need to)
0 means repeat forever, or until you stop it
1 means after 1 second
did -ra means remove all lines from the listbox and add the following line
MP3Player is the name of the dialog that you are using
1 is the listbox ID
$!insong.pos the $! makes the timer not evaluate $insong.pos until it's ready to run each time.