I was in the same situation, have the kit for the m7, so called HTC today. Was told it is not possible to buy the cradle separately, the kit is the same it is only the cradle that's different for each model. Kinda feel pissed about this. However I will sell my m7 kit to part fund the m8 kit. So it's not all bad.....
Still feel that HTC could offer the option of cradle only.
Sent from my HTC One_M8 using xda app-developers app