Hi,

Unfortunately, this is not a trivial modification and could not be covered by the free support offered by this forum. You may post your question in the official opencart forums, seek a freelancer to do the job for you or look for a third party extension.

Here are some useful links that could give you some clue:

http://forum.opencart.com/viewtopic.php?f=110&t=117665
https://qdappnet.wordpress.com/2015/05/09/how-to-add-custom-field-to-contact-form-in-contact-us-page-in-opencart-2-0/
http://stackoverflow.com/questions/29772576/how-to-add-telephone-field-in-opencart-2-0-2-0-contact-form

Best Regards,
ThemeBurn team