Schiff has stated he will arbitrarily approve or deny any request from the House republicans.
Today House republicans released a list of eight witnesses they want to see called to testify as part of the impeachment inquiry (full pdf below). Included in the list is the anonymous CIA “whistleblower”, Hunter Biden, Alexandra Chalupa (DNC operative who met with Ukrainian officials in 2016), and Nellie Ohr who was doing the Fusion-GPS 2016 opposition research using Ukrainian contacts and sources.
The full list includes: Hunter Biden, Devon Archer, Alexandra Chalupa, Tim Morrison, David Hale, Kurt Volker, Nellie Ohr and the “Whistleblower”. [Direct House pdf Link]
It is unlikely Chairman Adam Schiff would permit the republicans to call Nellie Ohr as a witness because her activity throughout the 2016 election would be too damaging to the current impeachment narrative. Indeed, questions to Nellie Ohr would likely expose the origin of the Clinton-DNC-Fusion opposition research and manufacturing operation that eventually blended with the DOJ and FBI.