Conversion is possible, but there's no automatic way to do it.
The attitude I almost always see is that converting between different editions of Dungeons & Dragons is generally possible, but there's no easy formula. You must apply a lot of judgement and make decisions yourself based on your knowledge of your target game edition. I can't find a product on the DM's Guild or otherwise which would assist.
The first document you want to read is, ironically, Conversions to 5th Edition D&D, since it gives some expert advice on the differences between AD&D and 5e, and you can use this information well when backporting things de 5e to AD&D.
The fifth edition of D&D is fairly compatible with adventures from the first, second, and third editions of the game—enough so that quick conversions of adventures from such editions are possible. Careful conversion is the alternative.
It notes, in particular:
- Encounter difficulty is more an art than a science. Compared to 5e, AD&D encounters tended to be on the easy side to allow for more combats and exploration.
- The number of monsters in an encounter matters less than the impression it gives.
- AD&D monsters tend to appear in great number and have fewer hit points.
- In AD&D, more treasure and magic items are given out than in 5e, especially from NPCs.
- 5e magic weapons cap out at +3, while in level AD&D you can see weapons above that.
I would add to this the following advice:
- Don't attempt to convert the adventure module wholly literally. Try to capture the sense of it. Consider that you are not traduzindo the adventure module, but rather creating a new module with the original as heavy inspiration.
- Don't convert the entire module, just the key parts that differ rules-wise (e.g. encounter composition). A perfect complete translation isn't necessary unless you're publishing it, and your players are unlikely to need exact stats for every NPC they meet. The broad strokes like story and map remain unchanged.
- It's your game, so the conversion is free to add, remove, or change entire elements in order to fit the sense of AD&D more.
- It helps to be familiar with both the edition you're converting from and the edition you're converting to, but the "to" edition is the most important.
- Most 5e monsters have a direct AD&D equivalent. Use the original stats for the AD&D monster (e.g. just use AD&D's ogre rather than trying to convert 5e's ogre stats).
- For monsters that don't exist in AD&D, an easy fix is to adapt an existing AD&D monster; e.g. give spells or special attacks to an existing monster which has approximately the right game stats, then change the description to whatever you like.