On PE5, you'd be looking at about 600m of line. As Chris said, what you can do is underlay it with another braid and then join the two lines together. After some recent testing of knots that I have done, I'd only use what Chris suggested which is two bimimis joined by a catspaw. If you're backing with mono, you could use something like a FG or a PR if you have the equipment to do one.-f the type of fishing you do is generally limited to fish where you will not need more than 300m of PE5, get yourself a 10000 spool and keep the 20000 spool for heavier line.
I was amazed to read an article from a very well known fishing writer from about a year to two ago suggest a 20000 with 50lb braid. You'd need a bloody serious spool of braid to fill this!