Is it possible to use external drive for time machine backup without full formatting?

So time machine asks for formating the drive and erasing all data; is there a workaround that would allow me to only format a part of it?

the drive is 1TB, my macbook pro is 128GB model, and the data currently on the external drive is about 200GB (it would be extremely hard to move it anywhere)

if you have tried this before or know of any resources that could help please share.

any help appriciated