When I back filled my fly reel I used the fly line backing, then fill it in with some 8# mono to smooth it out. Filled some 4# mono to better smooth it out smooth. Then I finally did my 2# mono line.
Yeah it took a little work, but I shaped the spool arbor into what I really wanted and made it smooth so that no little loop of 2# test will dig itself into anything.
Therein lies one of the challenges of super small diameter line even in reels with a so-call cross hatch laying of the line when retrieved. Larger diameter lines offer too many of these little hiding places for the thin stuff. Masking tape or cork makes for a smooth base to start winding on and will help prevent line digging far better than the next size of line even.
remove half of the heavier line already on the spool as lay a couple, three layers of masking tape right over it AFTER tying in the two pound. To make things easier, with the spool yet on the reel thread the new line thru the guides, tie the lines together then slip the line under the line keeper on the spool and apply the tape. When you have the tape in place, release the line from the clip and spool the line as you normally would. It maybe help to remove the spool from the reel to tape it.
Line of the same diameter simply finds its own level in use, but when smaller diameter line is laid on over a larger diameter line its too easy for the small stuff to find places to dig in as its base will continually re-adjust over the larger line base creating gaps for it to fall thru and into.
That 1000 series reel should use a tape of 5/8″ to 3/4″ and about 4″ should do it.