Is there a plugin for IDA Pro to enable me to add x64dbg debugger support to IDA Pro 6.8 or 6.9?
I am not aware of any fully-fledged plugin, but there are some halfway solutions.
Labeless looks promising and provides
Labels/Comments synchronization between IDA PRO and dbg backend (OllyDbg1.10, OllyDbg 2.01, x64dbg)
None of those provide a true debugger interface, but they do make life easier.