Assuming sd1.mysite.com and sd2.mysite.com are single WordPress installs.
You can create mysite.com as a WordPress Multisite, you can then export the posts, users, etc and reimport to the new multisite version of sd1 or sd2 using: https://en-ca.wordpress.org/plugins/wordpress-importer/
You will need to include the themes, plugins as well in the new multisite install.
If sd1.mysite.com and sd2.mysite.com are not single WordPress installs and just subdomains, you just need to point the subdomains to mysite.com IP address.
Edit:
Export the posts and import using the importer listed above. The sd1 and sd2 databases will no longer be needed as mysite.com DB now holds everything.
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