Modifying plugins

Hi all!
I am using the plugin User Profiles Made Easy and I have created a custom field. Now I don’t know if this custom field created can be visualized in the database or not, but what I want to do is to put some PHP function inside the custom field that checks if the number input by the user in the custom field exists in the database. Can someone guide me on how to do this? How can I and where do I put this custom code, modifying at the end the plugin.