Setting node ID sequence to a particular higher value for future created records
Yes, you can set the nid to a higher value in your D8 instance, then continue to migrate D7 nodes under that.
Something like the following to set the auto_increment value.
// Set the next auto increment field value on the table
$result = db_query('ALTER TABLE {' . $connection-&gt;escapeTable($table) . '} AUTO_INCREMENT = ' . (int)$auto_increment);

e.g for node and node_revision
// Set the next auto increment field value on the node tables
$result = db_query('ALTER TABLE {node} AUTO_INCREMENT = 8000000');
$result = db_query('ALTER TABLE {node_revision} AUTO_INCREMENT = 8000000');