You can use Nuxeo's extension system to do that just that. For instance you could create a file called

versioning-override-config.xml

in the following location:

<NUXEO_HOME>/nxserver/config

Then fill it with the following contents:

  <?xml version="1.0"?>
   <component name="org.nuxeo.ecm.platform.versionoverride" version="1.0">
     <extension target="org.nuxeo.ecm.core.versioning.VersioningService" point="versioningRules">
       <defaultVersioningRule>
         <initialState major="12" minor="0" />
        </defaultVersioningRule>
      </extension>
   </component>

and restart your server.

You can find more documentation and other possible applications of the versioning service: here

So you have two possible solution here.

You can filter the versioning rule per document if that's enough for you. Every document of this type will start with version defined in the versioning rule.

    <versioningRule typeName="File" enabled="true">
     <initialState major="12" minor="0" />
    </versioningRule>

This way every File document start with version 12.0 . You can always remove this contribution after the import.

Another solution would be to define your own versioningService. This way you can override the doPostCreate called every time a document is created by the method CoreSession#createDocument(DocumentModel) . The option Map it takes as parameter is the one from DocumentModel#getContextData . You can define your own versioningService with a contribution like this:

<extension target="org.nuxeo.ecm.core.versioning.VersioningService" point="versioningService">
    <service class="yourVersioningservice" />
</extension>

Hope it helps.