"Though" is already a word on its own, meaning "in spite of the fact that". It's a conjunction, not a base word that prefixes can be added to.
However, you could use "even" before "though" to emphasize the contrast:
* Even though it was raining, we still went for a walk.
Or you could use "although" as a synonym for "though":
* Although it was raining, we still went for a walk.