The instructions your linked to are for raidtools, so they would not go well with "while using mdadm" part of your request. I would locate mdadm specific instructions on how to accomplish what you want to accomplish.
You would be much better off backing up your data first before creating a raid, even if it's possible to create raid leaving 50GB of data in place (which I doubt, as mdadm needs to put a superblock somewhere on the drive which existing partition(s) will likely not allow to .. but I'm no expert here)
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