Importing customers can be do with the flow object.
Example of creating a customer:
<objects>
<object type="customer" external-reference="demo-1">
<lastname>Lastname</lastname>
<firstname>Firstname</firstname>
<email>test@domain.tld</email>
<passwd modifier="Tools::encrypt">myPassword</passwd>
</object>
</objects>
An other example of creating a customer plus a group and adding customer to the group:
<objects>
<object type="customer" external-reference="demo-1">
<lastname>Lastname</lastname>
<firstname>Firstname</firstname>
<email>test@domain.tld</email>
<passwd modifier="Tools::encrypt">myPassword</passwd>
<block>
<objects>
<object type="group" external-reference="demo-1">
<name lang="en">My Group</name>
<price_display_method>1</price_display_method> <!-- Taxes incluse -->
<block>
<objects>
<object type="customerGroup" external-reference="demo-1">
<id_customer>{{id}}</id_customer>
<id_group>\{\{id\}\}</id_group>
</object>
</objects>
</block>
</object>
</objects>
</block>
</object>
</objects>