Fasting on Monday and Thursday of a week is desirable and an established Sunnah. Besides other narrations also prohibit singling out Fridays for fasting. In your situation, you are trying to single out Thursday for fasting, which will not be preferable according to the general understanding of the prohibition of specifying a day for fasting, unless it is a Monday or Thursday. However, what appears to be the best alternative for you, to add one more day to the weekly fasting program, is what is known as the Fasting of Prophet Dawood AlayhiSalam. It is fasting on the alternate days, that is, every other day. A narration on the virtues of this type of fasting is:
The Prophet (peace be upon him) said to
Abd Allah b.Amr b. al-
As: “Fast one day and break your fast one day. This was the fast of David (peace be upon him), and it is the best of fasts.”
Abd Allah b. `Amr then said: “I can do better than that.”
The Prophet (peace be upon him) replied: “There is no fast better than that.” [Sahîh al-Bukhârî (1840)]
However fasting in this manner would mean that you have to miss some Mondays or Thursdays, but Allah knows the intentions of his slaves, He rewards what one does and what one intends to do from the good deeds.
Islam makes us conscious of the rights of our body and health, and therefore prohibits fasting on every day, and alternatively prescribes fasting on alternative days for persons who are keen to fast much possible days.
Here is good piece on combining the fast of Dawood and fasting on Mondays and Thursdays: