What is the past tense of word somnambulist?

The word "somnambulist" is a noun, not a verb, so it doesn't have a past tense.

To describe someone who sleepwalked in the past, you could use phrases like:

* was a somnambulist

* sleepwalked

* engaged in somnambulism

For example:

* "He was a somnambulist as a child, but grew out of it."

* "She sleepwalked through the house last night."

* "He engaged in somnambulism during his childhood."