I agree with Steve. Unless you are looking to significantly increase your NAS storage or use it for a number of other tasks then a roon core machine that either points at the storage on your existing NAS or has a USB disk attached sounds more sensible.
Those who have gone the NAS route tend to have a LOT of storage, which is why their NAS appliances are powerful enough to run roon.
Either wait for the roon Nucleus which will be released shortly Nucleus! when can we expect the first deliveries?
or buy a NUC and install it yourself - if you can cope with a NAS you can do this https://kb.roonlabs.com/Roon_Optimized_Core_Kit