Hi, you can create a new seatpost by putting two of them together. Other folding bikes like dahon which not only for children have (to) short seatposts too. The first seatpost should fit with the seattube of the frame and the second one with the inside measure of the first. Check this out by testing in a bicycle shop with different pairs of seatposts. From the first one the saddleclamp need to be cutted and slotted.
Its on your own risk. You can bridge with this method some missing centimetres. To much and the frame is getting damaged by breaking the seattube. Keep your attention on the minimum insertion and add some cm.



I did it in the same way and using the bike for many kilometres until now, it works.