As part of the plea agreement, prosecutors will recommend an eight-year federal prison term when he is sentenced in July, Roper said. He faced a maximum sentence of 10 years.
Larry Gordon May pleaded guilty to his role in the conspiracy in October and is to be sentenced in April.
The trial for Stephen Michael Ewing, also known as Stephen Michaels, is set for Monday.
The men used sham payroll companies with foreign addresses to evade taxes, according to court documents.
Trebert's attorney, Charles Blau of Dallas, said the evidence collected by prosecutors would have made trial difficult for his client, the Fort Worth Star-Telegram reported in Wednesday online editions. Blau said he hoped Trebert would testify against Ewing, whose attorney could not be reached for comment.