When you look at \Magento\Framework\Data\Collection\AbstractDb magento2 itself provide hook operation for your collection.

protected function _renderFilters()
    {
        if ($this->_isFiltersRendered) {
            return $this;
        }

        $this->_renderFiltersBefore(); // Hook for operations before rendering filters

....................
}

So, what you need to do this by just adding in your collection[NAMESPACE\MODULENAME\Model\ResourceModel\YOUR_CLASSNAME\Grid\Collection]

protected function _renderFiltersBefore() {
    $joinTable = $this->getTable('catalog_product_entity_varchar');
    $this->getSelect()->join($joinTable.' as cpev','main_table.entity_id = cpev.entity_id', array('*'));
    parent::_renderFiltersBefore();
}

When you look at \Magento\Framework\Data\Collection\AbstractDb magento2 itself provide hook operation for your collection.

protected function _renderFilters()
{
    if ($this->_isFiltersRendered) {
        return $this;
    }

    $this->_renderFiltersBefore(); // Hook for operations before rendering filters

    ....................
}

So, what you need to do this by just adding in your collection[NAMESPACE\MODULENAME\Model\ResourceModel\YOUR_CLASSNAME\Grid\Collection]

protected function _renderFiltersBefore() {
    $joinTable = $this->getTable('catalog_product_entity_varchar');
    $this->getSelect()->join($joinTable.' as cpev','main_table.entity_id = cpev.entity_id', array('*'));
    parent::_renderFiltersBefore();
}