No, You can not convert a jpg into a raw file.
But you can try converting into a DNG file.
DNG- is an open source RAW file format that was developed by Adobe.
Even you can not import a JPEG file in "Copy as DNG" mode, LR will prompt an error dialog saying, in substance: "Non Raw files are not converted to DNG." What you can do, however, is select your jpeg file in Lightroom, and export as DNG, and re-import that DNG file
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