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You can, but you would need to take it from a pre-paid service and put it onto a post-paid plan in order for it to appear on the same account.
If you wanted to keep the mobile as pre-paid, you could link it to appear with your post-paid services in My Account, here: https://tel.st/sl3js, which would give you the ability to administer both from the same location, but the pre-paid service would not appear on the post-paid bill.