Form validation problem with Opencart

I have a site e-commerce in opencart and I have a problem in the field RG which is as follows, when the user registers it fills all the fields normally but in the RG field there are those users who have one less digit in the RG they normally register in the site more when they are going to make a purchase they can not conclude it by the fact that the RG is with a number less would like to know if it is possible to put a code that does the following when the user registers a RG with a digit unless add a 0 in the field so that it is complete. I believe that this code goes inside the controller of the form I was not the one who developed the site but I have to make this change.