Dont know if this suits your need but its possible.
A simple way is to create a folder on your tvheadend server as a mountpoint for your nas storage:
sudo mkdir /local/directory/ -p
(this is the directoy you will configure in tvh to record storage)
sudo mount NAS_ip:/remote/directory/ /local/directory/
this is the basic, you may need to mount at start and adjust some previlegies.