but not to hesitate let's get into it. first, we need to locate the files in question, though if you already know this skip to number 7.
1. you need to right-click STS (slay the spire).
2. press the "properties" tap that comes up.
3. in the window that comes up press the "Local files".
4. in said tap press the "Browse".
5. now do the same in the "downfall Extention" tap.
6. you can also find them here (This computer > Local Disk > Program files(x84) > Steam > steamapps > common > SlayTheSpire/Downfall - A Slay the Spire Fan Expansion)
7. now in both of the explorer windows, you need to click the preferences folder.
8. in this folder, all of your STS unlock data is saved, just copy all of the files, in the SlayTheSpire subfolder, into the Downfall - A Slay the Spire Fan Expansion preferences folder.
9. now if you did this right you should have all your unlock data in the downfall explanation.

although I do not know if it will have an impact, I personally did delete the unlock files of other characters that belong to other mods. it is not needed just recommended.

ask if you have problems though i can't guarantee i will respond to them quickly