You can roll up any text field type. So just create a field of type formula text
ToText([Record Owner])
and then roll it up.
But then anticipating your next question, how the heck do I turn the block of rolled up text into a list user field?
var text value = ToText([Combined Text List-User(text format)]);
ToUserList(
ToUser(Trim(Part($value,1,";"))),
ToUser(Trim(Part($value,2,";"))),
ToUser(Trim(Part($value,3,";"))),
ToUser(Trim(Part($value,4,";"))),
ToUser(Trim(Part($value,5,";"))),
ToUser(Trim(Part($value,6,";"))),
ToUser(Trim(Part($value,7,";"))),
ToUser(Trim(Part($value,8,";"))),
ToUser(Trim(Part($value,9,";"))),
ToUser(Trim(Part($value,10,";"))),
ToUser(Trim(Part($value,11,";"))),
ToUser(Trim(Part($value,12,";"))),
ToUser(Trim(Part($value,13,";"))),
ToUser(Trim(Part($value,14,";"))),
ToUser(Trim(Part($value,15,";"))),
ToUser(Trim(Part($value,16,";"))),
ToUser(Trim(Part($value,17,";"))),
ToUser(Trim(Part($value,18,";"))),
ToUser(Trim(Part($value,19,";"))),
ToUser(Trim(Part($value,20,";"))))