Can I pull a new single conductor alongside an existing 14/2 wire for a fan+light install?
You cannot pull a single conductor.  (Retrofitting grounds is another story, that's allowed now.)

However, there may be "smart devices" to help you solve that problem, such as Infineon modules which which sit up in the fan/light and listen to electronic signals from a matching light/fan switch.  They are most commonly used on tavern style slow spinning ceiling fans, but I believe they make them for bathroom fans also.   Some also work with wireless remotes, smartphone and home-automation systems.