can I roast the nuts before I make a maple candied walnuts
I would recommend sticking to raw walnuts.  I recently prepared a batch of candied pecans and used raw pecans.  They turned out amazing and had sufficiently rich, roasted flavor.  Nuts can get bitter if they're roasted for too long. Roasting the nuts prior to putting them in the maple candied walnuts recipe might lead to over-roasting.

My recipe called for tossing the pecans in a one egg white (to develop a super thin, sticky coating) and then sprinkling them with cinnamon sugar before roasting.  They turned out amazing.  I've been snacking on them non-stop! Good luck!