Is it possible for a digital multimeter to be modified to read 10 times the ac voltage?
You can use a transformer to step down the voltage. You can use a transformer with spec Vout/Vin = 10, in which case your multimeter will read 240 volts as 24 volts and 1 volt as 0.1 volt. The resolution will be an issue here, but if you don't need high precision, you can use this method. However, DC level shift will be brought to zero as transformer does not work in case of DC voltages. 

Hope this helps.

Mohit