there are no such tool (and a short google search confirms that) but in most cases the migraiton can be done by hand since the squid.conf has pretty readable format which contain general options (proxy ports, auth settings), ACL elements, and access lists. Fortunatly both Squid and MWG have the same rule based approach, that allows nearly 100% migrations.
Check this squid FAQ: https://wiki.squid-cache.org/SquidFaq/SquidAcl and if you still have troubles to convert some parts of the squid.conf to MWG rules you can post you can post (anonymized) excerpts from the squid.conf file here to get help with migration.
If your squid.conf contains thousands of ACLs you can ask somebody to write a script to do the work.
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