Yes, it is permissible to offer voluntary (nafl) prayers while sitting even without an excuse, though it is better to stand if able. The Prophet (peace be upon him) said that the one who prays sitting receives half the reward of the one who prays standing, as reported by 'Imran ibn Husayn. 'A'ishah reported that in his later years the Prophet prayed much of his voluntary prayer while seated. Note that this half-reward applies to voluntary prayer for one who is able to stand; a person with a valid excuse who cannot stand receives the full reward, and obligatory prayers must be prayed standing for whoever is able.
Evidence — the daleel
Sahih al-Bukhari 1115 ṢaḥīḥSahih al-Bukhari 1116 ṢaḥīḥSahih Muslim 735 Ṣaḥīḥ
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.