You can Call another Grid Via Two Ways. Hope You know the whole structure of Magento Grid Management.

Step 1: Open your Module Folder like [NameSpace]\[Module Name]\Block\Adminhtml\[modulename]\Edit\Tabs.php.

Step 2: In _beforeToHtml() Method add last Below Code.

  $this->addTab("Custom Block", array(
        "label" => Mage::helper("custom")->__("Custom Block"),
        "title" => Mage::helper("reordermanagement")->__("Custom Block"),
        'content' => $this->getLayout()->createBlock('[modulename]/adminhtml_[anothermoduleanme]_grid', 'custom-tab-content')->toHtml(),            
        'class' => 'ajax'
    ));

Step 3: This is one Way to add the Custom grid to another tab grid.

Or

Step2: Another Way you have to create a action in controller.

Step 3: Add Below Code.

$this->addTab("reorder History", array(
        "label" => Mage::helper("reordermanagement")->__("Reorder History"),
        "title" => Mage::helper("reordermanagement")->__("Reorder History"),            
        'url'=>$this->getUrl('*/*/grid', array('_current'=>true)),
        'class' => 'ajax'
    ));

Step 4: Cerate gridAction() Method in [Namespace]\[Module Name]\controllers\Adminhtml\yourController.php create Below Method.

 public function gridAction()
 {
    $this->getResponse()->setBody(
        Mage::app()->getLayout()->createBlock('[modulename]/adminhtml_[anothermoduleanme]_grid')->toHtml()
    );
}

This are simple steps to add Grid into Custom tab.

You can Call another Grid Via 2 Ways. Hope You know the whole structure of Magento Grid Management.

Step 1: Open your Module Folder like [NameSpace]\[Module Name]\Block\Adminhtml\[modulename]\Edit\Tabs.php.

Step 2: In _beforeToHtml() Method add last Below Code.

  $this->addTab("Custom Block", array(
        "label" => Mage::helper("custom")->__("Custom Block"),
        "title" => Mage::helper("reordermanagement")->__("Custom Block"),
        'content' => $this->getLayout()->createBlock('[modulename]/adminhtml_[anothermoduleanme]_grid', 'custom-tab-content')->toHtml(),            
        'class' => 'ajax'
    ));

Step 3: This is one Way to add the Custom grid to another tab grid.

Or

Step2: Another Way is to create a action in controller.

Step 3: Add Below Code.

$this->addTab("reorder History", array(
        "label" => Mage::helper("reordermanagement")->__("Reorder History"),
        "title" => Mage::helper("reordermanagement")->__("Reorder History"),            
        'url'=>$this->getUrl('*/*/grid', array('_current'=>true)),
        'class' => 'ajax'
    ));

Step 4: Create gridAction() Method in [Namespace]\[Module Name]\controllers\Adminhtml\yourController.php and create Below Method.

 public function gridAction()
 {
    $this->getResponse()->setBody(
        Mage::app()->getLayout()->createBlock('[modulename]/adminhtml_[anothermoduleanme]_grid')->toHtml()
    );
}

These are the simple steps to add Grid into Custom tab.