“Accomplishing the impossible means only that the boss will add it to your regular duties” is a jocular work saying that has been printed on many images. American newspaper columnist Doug Larson (1926-2017) wrote this in his column in the Green Bay (WI) Press-Gazette on April 30, 1987.
   
   
Wikipedia: Doug Larson
Doug Larson (February 10, 1926 to April 1, 2017) was a columnist and editor for the Door County Advocate (1953–1964) and wrote a daily column, “Doug’s Dugout,” for the Green Bay Press-Gazette (1964–1988), both Wisconsin-based newspapers. The column was originally syndicated through United Media under the title “Senator Soaper Says”; Larson took over authorship in 1980. Previously, it had been written by Bill Vaughn of the Kansas City Star. Larson was born in Sturgeon Bay, Wisconsin.
   
30 April 1987, Green Bay (WI) Press-Gazette, Doug Larson column, pg. A-13, col. 5:
Accomplishing the impossible means only that the boss will add it to your regular duties.
         
27 June 1987, The Leaf-Chronicle (Clarksville, TN), pg. 4A, col. 1:
On the Light Side
Accomplishing the impossible means only that the boss will add it to your regular duties.
   
Google Books
A Rainbow of Hope:
777 inspirational quotes plus selected scriptures
By Billy Hughey and Janice Hughey
El Reno, OK: Rainbow Studies, Inc.
1994
Pg. IV:
Accomplishing the impossible means only that the boss will add it to your regular duties.
Doug Larson
   
Google Books
Words from the Wise:
Over 6,000 of the Smartest Things Ever Said
By Rosemarie Jarski
New York, NY: Skyhorse Publishing
2007
Pg. ?:
Achieving the impossible means only that the boss will add it to your regular duties.
Doug Larson
 
Latin Times
Administrative Professionals Day 2016 Quotes: 28 Sayings To Show Appreciation
By Maria G. Valdez | Apr 22 2016, 02:51AM EDT
(...)
“Accomplishing the impossible means only that the boss will add it to your regular duties.” - Doug Larson
 
International Business Times
Administrative Professionals Day 2017 Quotes: 10 Ways To Say Thank You To Your Assistants
By Ishani Roy @royishani_roy
04/25/17 AT 6:00 AM
(...)
6. “Accomplishing the impossible means only that the boss will add it to your regular duties.” — Doug Larson
   
Tuko
20+ hilarious quotes about work
Author: Venic Nyanchama UPDATED: January 8, 2019
(...)
Accomplishing the impossible means only the boss will add it to your regular duties.
 
Twitter
Cameka Smith
@iamCameka
Accomplishing the impossible means only the boss will add it to your regular duties.   
Doug Larson #BOSSQuotes
8:00 AM - 8 Apr 2019