Does anyone knows how to estimate the motor contribution to short circuit capacity when adding a VSD to the system, if the drive is a current-source inverter or a PWM with regenerative capabilities.

The current limiting reaction time of the VSD output should far less than the time constant of the bus caps. Any short circuit requiring external protect or capacity should be from a failure in the VSD input rectifier circuit. The standard VSD fuse will be class J and the are many charts for sizing.