Magento: Add custom dropdown in adminhtml form

In your Namespace/Module/Block/Adminhtml/your_module/Edit/Tab/Form.php
Collect your collection

// product collection dropdown start
			$category = Mage::getModel('catalog/category')->load(3);
			$productCollection = $category->getProductCollection();
			$productCollection
					->addStoreFilter()
					->addAttributeToFilter('type_id','configurable')
					->addAttributeToSelect('*');
			foreach($productCollection as $_product) { 
        if($_product->getParent == NULL){
            $_product_dropdown[] = array(
							'value'     => $_product->getId(),
							'label'     => $_product->getName(),
            );
        }
			}

$fieldset->addField('product_id', 'select', array(
			'name'      => 'product_id',
			'label'     => Mage::helper('warranty')->__('Product Name'),
			'title'     => Mage::helper('warranty')->__('Product Name'),
			'required'  => false,
			'note'      => $note,
			'class'     => 'HideIt',
			'values'    => $_product_dropdown,
		));