Is it possible to use words as starting and ending points of excerpt parameters?

I think this is more a PHP than a WP question. Try using a regular expression with preg_match:

$theExcerpt = get_the_content();
$returnValue = preg_match('/(string01)(.*)(string02)/', $theExcerpt); 
echo $returnValue[0];

You have to replace string01 and string02 with your ‘begining of paragraph’ and ‘end of paragraph’. There might be a more elegant way of using preg_match, but I think this will do the trick.

Don’t forget to escape the output