Step 1). Get the Product image from URL – create a file in Helper/Data.php

public function getProductImage($imgurl){
	 		$fileSystem = $this->_objectManager->create('MagentoFrameworkFilesystem');
	        $mediaPath = $fileSystem->getDirectoryRead(MagentoFrameworkAppFilesystemDirectoryList::MEDIA)->getAbsolutePath();
	        if (!file_exists($mediaPath.'imagefolder')) {
	            mkdir($mediaPath.'imagefolder', 0777, true);
	        }
	        $url = $imgurl;
	        $ch = curl_init($url);
	        $my_save_dir = $mediaPath.'imagefolder/';
	        $filename = basename($url);
	        $complete_save_loc = $my_save_dir . $filename;
	        $fp = fopen($complete_save_loc, 'wb');
	        curl_setopt($ch, CURLOPT_FILE, $fp);
	        curl_setopt($ch, CURLOPT_HEADER, 0);
	        curl_setopt($ch, CURLOPT_TIMEOUT, 28800);
	        $return = curl_exec($ch);
	        curl_close($ch);
	        $image = $return;
        	return $image;
        }

Step 2). Assign the images to existing products.

$product = $this->productFactory->create()->load($proid);
	$getimgurl = $url;
	$imagename= basename($getimgurl);
	$imgpath = $mediaPath.'imagefolder/'.$imagename;
	if (!file_exists($imgpath)) { //check image already exist in directory
	    $this->helperData->getProductImage($getimgurl);
	}
$product->addImageToMediaGallery($imgpath, array('image', 'small_image', 'thumbnail','media_gallery'), false, false);
$product->save();