Can it be done on a Mk 6.5 Fiesta ST?

In a word no sorry. Its one of those things that if you want it it HAS to be fitted when the car is being manufactured.

Yes have to be fitted on the car when been built! sorry!

There's nothing you can't do actually, if you've got the time and money!!
Phone the ford technical helpline if you have not got access to ETIS to see if the wiring harness supports ESP.
The Fiesta ST may have a one spec harness so the optional stuff you haven't fitted may have the connections in place. If it hasn't then you will need to source an engine bay and cabin harness from an ESP spec car.
You will need to fit a steering angle sensor on the steering wheel, not sure where it is on Ford's but on the Jags its directly behind the steering wheel, the Jag sensor is a Ford sensor so it's probably the same.
You will need a YAW rate sensor, these are always in the centre of the car, so I would imagine its under the console.
You will need a new ABS module that supports ESP, a new brake booster because ESP also has emergency brake assist and probably an engine ECU because ESP also has traction control. Oh and also a deactivation switch for the dash.
Once you'd fitted everything you would then have to have the car flashed to tell it it's an ESP car.
Also, ESP cars have the felex pack which is side and curtain airbags so you will probably need to change the seats and the Aibag module before you can Flash the car.

With brakes being such an important safety feature of the car, I really wouldn't recommend this conversion though.
It would probably be cheaper to order a new ST with everything on.