Hi,
When you rectify the AC voltage to DC and add filter capacitors, you have a DC voltage that is approximately 1.414x the AC input. So, you have a 1.414 * 240 = 339VDC supply.
Use a DC-DC converter now to convert this to 240VDC. You can use TOPSwitch from Power Integrations, VIPERswitch from ST Microelectronics or a discrete circuit with UC384x and MOSFET.
An alternative is to drop the 240VAC to 170VAC using a transformer and then rectifying this to DC and filtering it. So, you will have a 170 * 1.414 = 240VDC supply.
The latter method would be much easier, but maybe more expensive / bulky, depending on your power rating.
The first method would be a little trickier, but not very difficult nonetheless, and will be lightweight and cheaper, but again this depends on your power rating.
Hope this helps.
Tahmid.