PIL doesn't support this but you can fake it: Render the text four or eight times with the outline color using one pixel offsets:
x+1,y
x-1,y
x ,y+1
x ,y-1
(four times version)
x+1,y+1
x ,y+1
x-1,y+1
x+1,y
x-1,y
x+1,y-1
x ,y-1
x-1,y-1
(eight times version)
and then once at x,y
with the fill color.