There's no way to customise the Performance Policy used within the vCenter plugin for datastore creation from anything other than VMware. Reason being that a datastore can contain one or more applications of various block sizes, and thus the array volume block size needs to be set at the lowest common denominator to ensure no block misalignment or performance issues. If the policy was set to Exchange, then that would set the block size to 32k block alignment - however if anything other than Exchange was to be placed in that datastore there could be space efficiency and/or performance issues.
If you wish to create a datastore just for Exchange then either create one via the Nimble GUI/CLI, or use VVOLs - which will give you this application granularity without the requirement for datastore overheads. The latter is my preference!
I want to know if it is possible to change the performance policy when creating a Datastore/Nimble Volumes through Nimble Connection Manager other than going back in Nimble Gui and change it there; by default it has VmWare 5 performance policies applied and for some I want to applied Exchange Policy