Can I use a seat-post clamp/collar on the seat-post itself to prevent slipping? I have a Cannondale Synapse (2015 model), and have swapped the stock Cannondale seat-post for a Thomson Elite inline seat-post. However, I am experiencing problems with the seatpost slipping, even when tightening to 5Nm, as specified on the Cannondale frame. The seatpost clamping mechanism is an internal two-wedge system.

The frame is carbon, and the seatpost is aluminium. I am using carbon paste, but this does not seem to have truly solved the problem.

I am considering adding a Thomson seat post collar directly onto the seat post, but am nervous about adding extra downward stress onto the carbon frame.
You would be best served by first going to you dealer for advise. I am unsure why you would change the seatpost, as this may provide more salient information regarding your problem.

That said, if you really want to use the seatpost mentioned, it sounds undersized. You can buy seatpost specific shims. I again would recommend going to a LBS as they have the ability to best gauge the correct size shim, and order it for you. CF frames do not handle stresses that they weren't designed for well.