Is it possible to replace cylinder locks with 5 lever mortice locks ?

If it is a wood door it is possible, I wouldnt recommend on any other type of door although it is possible. If it is the security you are concerned about with cylinders have you considered anti snap high security cylinders

Yes, on wooden doors most people opt to keep the cylinder lock and as added security they also add an additional deadlock

No it is not possible, the inside of the door consists of a mechanism which usually has a 5 locking point system consisting of two hooks, two rollers, and a deadbolt. A mortise lock only provides a deadbolt and the hole which has been made to fit the mechanism will be bigger then the mortise lock which you require to be put in which makes it unable to fit as there is nothing to fit it to. If you are concerned about security then the best advice I can give is upgrade the barrel to a 3star cylinder. This will provide the best level of security possible, they are really good. Additionally, sash jammers can be fitted to the frame of the door to give the door a tighter fit which can stop the door from dropping over time and also act as an additional security feature to the door. These sash jammers also are available with locks on. I hope this information helps.

You dont mention if its a UPVC door with multipoint locking (mpl) or a wooden door. If the cylinder is part of a multi point locking system, ERA make a gearbox with a 5 lever locking. It may be a cylinder locking on a mortice lock, this can be swapped easily for a B/S mortice with slight wood working. You can add a good quality locking latch and staple to a UPVC door fitted with a MPL giving additional security but it is quite a complex job for DIY and best carried out by a professional.