Holmes will testify alongside Fiona Hill, a former White House Russia adviser, in the week's final hearing.
With the addition of Holmes, the House intelligence committee is hearing from nine witnesses publicly this week.
In an interview Sunday with CBS "Face The Nation" host Margaret Brennan, House Speaker Nancy Pelosi assured the American people that congressional investigators are "unfolding the facts", but emphasized that so far those facts are proving President Donald Trump's guilt.
Multiple witnesses overheard a phone call in which Trump and Sondland reportedly discussed efforts to push for the investigations.
"If Donald Trump doesn't agree with what he's hearing, doesn't like what he's hearing, he shouldn't tweet".
Mr Trump's July 25 phone call is at the heart of the House of Representatives' Democratic-led inquiry into whether the Republican president misused U.S. foreign policy to undermine former Democratic Vice President Joe Biden, one of his potential opponents in the 2020 election.
Pushing back against accusations from the Republican President that the process has been stacked against him, Ms Pelosi said Mr Trump is welcome to appear or answer questions in writing, if he chooses.
Pelosi argued against the "dangerous position" that the election ought to decide Trump's fate rather than the House impeachment investigation, saying that Trump "is jeopardizing the integrity of the 2020 elections".
All eight witnesses will appear before lawmakers ahead of a weeklong recess for Thanksgiving.
Previous testimony suggests Sondland was involved in conversations key to the investigation into whether Trump held up military aid to Ukraine in exchange for investigations into Joe Biden and his son Hunter.
Tim Morrison, the top Russian Federation official on President Trump's National Security Council, is scheduled to testify publicly this week after a transcript released in recent days from his closed-door session appeared to contain versions of a number of events that differed from those of Gordon Sondland, the US ambassador to the European Union.
"Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released statement from Ukraine", Trump said. Republicans control the Senate and have shown little support for Trump's removal.
"Our insane, Do Nothing (where's USMCA, infrastructure, lower drug pricing & much more?) Speaker of the House, Nervous Nancy Pelosi, who is petrified by her Radical Left knowing she will soon be gone (they & Fake News Media are her BOSS), suggested on Sunday's DEFACE THE NATION that I testify about the phony Impeachment Witch Hunt".