And it would close the system for up to three years.
As part of the current plan, Strathclyde Partnership for Transport examined all 15 stations on the Glasgow network to find out if it could provide disabled access to platforms.
But the work would cost the same as the already agreed plan, making a total of £600m.
Strathclyde Partnership for Transport has begun the £300millon modernisation of the Subway network which will be finished in 2020. Work will include driverless trains, upgraded stations and smartcard tickets similar to London's Oyster card.
The 115-year-old under-ground carries more than 10m passengers each year and transport experts say the city would grind to a halt if it shut.
As a result, lifts will be installed at only two stations.
Director of projects Charlie Hoskins, said: "We have put an enormous amount of work into accessibility at the Subway and I am pleased to say we are able to instal lifts at St Enoch and Govan.
"We have had members of the public saying if we are spending all this money why are we not making the stations more accessible.
"However it is physically and operationally impossible as the whole Subway would be shut for two or three years and it would cost an extra £300m to do."
SPT has agreed to spend almost £400,000 on water-proofing work on a section of the underground tunnel between Kelvinhall and Patrick stations.
It is regarded as the worst part of the network for water ingress as a result of old coal seams and fractured rock.
The work is part of a £25m project to improve tunnels, brickwork and the track bed.
SPT chairman Jonathan Findlay said: "This is a significant amount of funding and it will be targeted at improving the worst hit area of our Subway tunnels for water ingress.
"Our Subway modernisation programme is not just about stations and trains but about ensuring infrastructure – and that includes our tunnels –can stand the test of time for generations to come.
"Keeping the Subway operation is key and that is why work to inject tunnel linings with water proofing materials will happen through the night making sure passengers travelling through the day are not affected."