Hi Myley and welcome to the forum.
The Sims 3 and Sims Medieval are two different games when it comes to programming. Because of these different programming/coding issues, it is not really possible to 'transfer' items from one game to the other.
There are those folks out there that create and/or try to create items that are similar to the furniture/patterns in Medieval for the Sims 3 but this 'custom content' (as it is called) is to be used at your own risk but not recommended by this forum.
Then there are those that try to change the game coding to create usable items like the poison you mentioned. These items are called 'Mods' are definitely not recommended for use and are actually forbidden to be named or talked about on this forum.
Just think - if the person didn't know what they were doing, then an item they made/created could corrupt your entire game.
EDIT - samoht04, we think alike but your answer is simpler
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