No, there is no insert/shim for the handlepost.

If you want to use an extra clamp on the adjustable handlepost, a 31.8/32.0 seatpost clamp can fit at the top part of the lower-handlepost section. (It will not fit lower down, since with most clamps, they have a lip at the top that normally keeps them in place on the top of your seatpost tube... unless you file that lip off, or find one without such a lip.) In this way you can fit an additional clamp on there. The normal Dahon handlepost clamp is about 1 inch down the lower-section. I have actually removed by Dahon clamp altogether in favour of a 3rd party clamp, though I would not recommend this unless you are counting grams.

I am using an Extralite 31.8 clamp at the moment, although a better choice might be the Salsa clamps (32.0) as it is a more robust clamp and I felt it fit more securely.