There is a plugin for this. It is Network Username Restrictions Override. Please remember to check out the FAQ #2 to know how to allow periods in usernames. Let me quote it here…
You’ll have to update the regular expressions in your .htaccess file
if you use email addresses or periods in site URLs. The default
WordPress .htaccess configuration does not allow them, nor will this
plugin edit your .htaccess automatically due to the risk involved.
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