"The Tirkel Commission invited me at the start of the testimony but I refused and I sent a letter clarifying my refusal," Zedan told AFP.
"I clarified that I do not want to cooperate with this committee because it was appointed by the government, which is a party (to the dispute)... I view it as empty of meaning and its recommendations as unacceptable."
Both he and Abu Daabis changed their minds after being warned by Israeli authorities that they were required by law to attend, they told AFP.
The pair are due to testify on Monday at 1000 GMT.
The four-man Israeli commission, headed by former Supreme Court judge Yaacov Tirkel and joined by two foreign observers, has heard testimony from Prime Minister Benjamin Netanyahu and Defence Minister Ehud Barak but has not been allowed to question any of the troops involved in the raid.
Israeli military Chief of Staff Gabi Ashkenazi testified before the commission for a second time on Sunday.
He reiterated that soldiers opened fire on the Mavi Marmara only after they were attacked by scores of knife- and club-wielding activists and believed their lives were in danger, adding that they tried but failed to clear the deck with sound grenades.
"I think somebody waving an axe at your head is a clear and present danger, and we had soldiers who received axe blows to the head," he said, according to the official transcript of the hearing.
The activists have said the soldiers opened fire as soon as they landed.
The deadly raid sparked international outrage and led Israel to ease a four-year blockade of Hamas-ruled Gaza to allow in all purely civilian goods as well as construction materials for internationally supervised projects.
The incident also strained once-close ties between Israel and Turkey, which has demanded an apology and an international investigation into the incident.
The Tirkel Commission has invited Turkish citizens who were on board the ship to testify but none have responded, said commission spokesman Ofer Lefler.
The UN secretary general is carrying out a separate investigation into the raid, and the UN Human Rights Council earlier released a report accusing Israel of violating international humanitarian and human rights law.
Israel rejected the latter probe from the outset as "biased."
Israel's 1.3 million Arab citizens comprise 20 percent of the population, and are made up of Palestinians who remained in the country following the establishment of the Jewish state and their descendants.