Converting a .jar file (Java Edition mod) to an .mcaddon file (Bedrock Edition) is a complex process because the two versions of Minecraft use entirely different coding languages and engines. While there is no universal one-click tool to perfectly convert every mod, specialized tools and manual steps can help port specific assets.
Extract textures from the JAR. They usually fit Bedrock, but may need resizing to power-of-two dimensions (e.g.,
If you are searching for a website where you upload a .jar and download a .mcaddon —stop looking. It does not exist because the code languages are not compatible.