Nowdays most game developers have allowed for modders, to use some contents from their games (models and textures mostly) as long as any usage is non-commercial, non-profit and doesn't have a negative effect on their product or company.
Here on this link, you can find developers who have allowed it.
Click on the developer name in order to read the offical response. Those who are not on the list are a NO, NO.
WHEN COMES TO USING/PORITING FROM ANY MOD INTO YOUR MOD. THEN YOU ARE OBLIGATED TO ASK ORIGINAL MOD MAKER FOR A PERMISSION TO USE THEIR CONTENTS. IF THEY SAY NO, THEN YOU ARE TO OBEY AND RESPECT THEIR DESICION.