You can create you own programmatic shape by just deriving your class from Telerik.WinControls.ElementShape class. But let me first say that the RoundRectShape roundness can be adjusted. If you are using the Visual Style Builder (VSB) application you can specify the edge radius, typing the following, for example, for the value of the Shape prop:
The "|" stands for a property value separator and the number "20" for the radius value.
Using this code, the shape radius parameter can be passed on the constructor. Using design time you can specify the property Radius of the shape object.
Here is also some info about custom shapes in the VSB application. Currently the VSB would not allow you to select from a list with custom shapes, because it only loads shape types found in the Telerik.* assemblies. We are working on improving this functionality and in case of anyone is interested, we can discuss it.