Best to make a voltage divider using a string of high value resistors. First, you need to find out the input resistance of the multimeter. Often this is 10M ohm. See the instruction book.
If it is 10M ohm, then a string of resistors totalling 90M ohm in series with the input will give a 10:1 range increase (0 - 6kV).
Good idea to build all the resistors inside a plastic box. Even better, fill the box with potting compound.
Be very careful here. If the ground probe gets disconnected then the meter will be at high voltage!