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As far as I know, in most states the significant other would have to legally adopt, or become the legal gardian of the child in question in order for that child to be classified as "dependant" on the significant other. Your significant other cannot approve medical procedures for the child, thus they cannot offer insurance on said procedures.
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The short answer is no. Unless that significant other was the spouse, he could not extend coverage to the child.
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In my case, I am the noncustodial parent & was order to carry medical coverage for my son. When I got married my wife was able to put my son on her insurance plan as well.
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I have my stepdaughter on my health insurance. My insurance company allows it for children that live with us. The insurance would also cover children that don't live with us but he is court mandated to provide insurance for them.