Use XML Layouts to set your custom template for product grid container block and add your custom form block there. You need to extend adminhtml_catalog_product_index
layout handle for that:
<adminhtml_catalog_product_index>
<reference name="product_list">
<!-- Set your custom template -->
<action method="setTemplate"><template>path/to/your_template.phtml</template></action>
<!-- Add your custom block -->
<block name="import_form" as="import_form" type="your_module/form_block_name"></block>
</reference>
</adminhtml_catalog_product_index>
Then you need to define your block and template. Your custom block should be extended from Mage_Adminhtml_Block_Widget_Form
and template should be a copy of adminhtml/default/default/template/catalog/product.phtml
but with modifications to display your custom block, like in the following example:
<div class="content-header">
<table cellspacing="0">
<tr>
<td style="width:50%;"><h3 class="icon-head head-products"><?php echo Mage::helper('catalog')->__('Manage Products') ?></h3></td>
<td class="a-right">
<?php echo $this->getButtonsHtml() ?>
</td>
</tr>
</table>
</div>
<!-- Start of Displaying of your custom import form -->
<?php echo $this->getChildHtml('import_form');?>
<!-- End of Displaying of your custom import form -->
<?php if( !$this->isSingleStoreMode() ): ?>
<?php echo $this->getChildHtml('store_switcher');?>
<?php endif;?>
<div>
<?php echo $this->getGridHtml() ?>
</div>