Anything is possible with some work. All depends on how the washer is run on the old one. New one is what, two wires? Has to be one wire going to the old pump to "tell" it when you have depressed the wash button, you would just have to run that to the washer pump, and a hose up to the squirters. You are right, the in tank ones are easier to change, and more reliable to boot. Sorry I don't have specifics on the change, I used a newer wiring harness so i didn't have to study up on the washer circuit.
Dorian
My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>