Answer
Yes, it will be permissible to sit and read your Nafl Salaat even if you are not ill and you have no excuse for sitting. (Shaami vol.2 pg.34; HM Saeed)
However, a person will get the full reward if he reads the Nafl Salaat standing. For sitting and reading Nafl Salaat without any excuse, he will get only half the reward. (Fataawa Mahmoodiyah vol.2 pg.128)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai