> would it be possible (and not too complicated) to turn this into a +/- 15

v psu

The main problem is that you will need another transformer (~ 2x18V instead
of 2x15V) as there is not sufficient "headroom" for a perfect 15V supply
(i.e. without ripple noise). And two resistors have to be changed which
define the output voltage range of the trimming potentiometers.

Best wishes
Dieter Doepfer