3 phase to single phase UPS 10 kva
I have a 3 phase to single phase UPS 10 kVA [note capitals]. 


OK. Why not tell use the make and model and give a link to the datasheet?


  The input voltage required is 332 V to 444 V / 3 phase voltage with an output of 220 - 240 V single phase. 


OK.


  I have an external battery bank of 2 V x 12 connected in series to make up 24 V.


Why not tell us the Ah rating and maximum discharge current of the cells? That will be crucial.


  My D.C. Voltage is still not enough to run the ups inverter when my 3 phase supply goes off. 


What voltage does it need?


  Can I be able to get a D.C. To D.C. converter from 24 V to 400 V with a high amperage. Instead of adding more batteries.


Sure you can. Will it be efficient? Probably not. Will it work? Maybe.

To get 10 kVA at 24 V would require \$ I = \frac {P}{V} = \frac {10000}{24} = 400 \ \text A \$ at 100% efficiency. The actual current will increase inversely with decreasing efficiency.