if your existing shower has 6mm cable and you have just run in a 10mm cable then providing the jb is suitably rated then yes you can.
HOWEVER if your shower has 10mm cable and you want to join it to a 6mm cable then this implies the shower needs a 10mm cable, so no you can not. because the 6mm cable will not be able to carry the amount of current required safley.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local