Hi @Zafer, welcome to the forum 
SubConn Micro Circular bulkheads with 2-8 contacts use a 7/16" thread (~11.1mm diameter), which is indeed too large to fit through the 10.2mm holes in our end caps.
Beyond that, the ROV Power Supply Enclosure contains power electronics that are used for high voltages, so unless you’re trained to work with high voltages it’s not recommended to open or modify that enclosure.
Given it’s a custom end-cap, there’s likely not much we (Blue Robotics) can do to help. Perhaps @Outland can provide some suggestions on possible courses of action.