You have to copy everything for one design except the name, town, and creator data since that data is different in New Horizons compared to New Leaf. Then I simply overwrote the design data of one of the default pro designs with the data from the .acnl file making sure to only write data where the design is and not the name, island, and creator data. Then I simply go to where the utf-16 text is in the hex editor for the name, island, and creator and change the name of it to the name of the design being imported but keeping it the same island name and changing it from "someone" to my own name that way the design can still be edited in game. It is important to make the design editable in game because of the way you're importing from a New Leaf clothes design into a New Horizons pro design so you can correct the way the design is laid out over the clothes and fix the color palette by switching to a color palette that is as close as possible to the color palette of the original design and then you have to make corrections to the color palette by hand and comparing with your own eyes. This is definitely the pain in the ass method. I have had success so far for a few pro designs but it takes a while and a lot of patience. It's either this method that takes a long time - due to all the hex editing and frankensteining data so it's recognizable by the game as a design in the save data and all the in game corrections that have to be made - or waiting for all these devs to finish adding pro design support after all the other features they are pushing out first. This is obviously not as easy as using some tool and requires some understanding of hex editing save data and figuring out how the game will recognize it so it doesn't import a design wrong. Keep in mind also this is exclusively New Leaf to New Horizons importing as anything made in New Horizons and shared online is going through Nintendo's Online system and isn't as easy to transfer to a console that doesn't have Nintendo Switch Online. We'd have to get a daring person with a Nintendo Switch Online subscription who has received all sorts of cool patterns to willingly put their Online subscription at risk by modding their console potentially sending out telemetry just to dump their save with all the pro designs from people online for those that don't have the Online subscription or have a banned console. I heard a rumor that getting cloud saves from a non modded console to a modded console might be possible but that still requires more research.
Apology for responding late to this, but you mean like copy it like this;
oh my th-that would be a very stupid thing to do xD not worth it for the person to lose their system being banned not unless they do not care at all. anyway thank you for telling me this..i mean i am copying the junko outfit from happy home designer ^^;; so i believe that maybe the problem here....sigh cause it won't load onto the game or show at all. but yes waiting for the people to import the pro designs is gonna take a while x.x"...sigh that's why i wanna do it this way which of course is gonna be a possible pain in the ass.