Yep, done that before with my braid except I don't use backer to start with and use 20 & 30# so it does use more spool depth to start. I keep empty line spools and use a small bolt thru 'em as a drill arbor. Chuck it up in cordless drill and spool off of reel onto empty filler spool. Since this leaves the "new" end on top, I chuck up a second spool and then run it off the first spool onto the second so I can then retie the "old" end onto the reel spool and load it with the "new on top.
Never did that with a spinning rig - just baitcasters.
It's a bit more time-consuming but I do this in the winter when the water is hard anyway and gave me something to do. I found that using the small filler spools are a better choice since they're obviously closer in diameter to the reel spool and don't make the reel sing such a squeal while emptying it.
Btw; I suggest using a variable speed drill at low speed to do this. Don't ask why - I'm sure you can imagine.
