As far as I know this is not possible 
Block connection checks are encoded by the Blockly.Blocks.Utilities.YailTypeToBlocklyTypeMap which is not accessible to extension developers.
If the type you want to create is an enum, support for custom dropdown blocks will be available after my GSoC project is merged. So in that specific case we're working on a solution 
But generally this is not possible I'm sorry I can't be more help.