How can I add data in a list from some other field?

I HAVE A DROP DOWN LIST WITH NAMES OF FIRMS. HOWEVER IF A FIRM IS NOT IN THE LIST USER SELECTS "NOT IN THE LIST"AND HAS TO ADD MANUAL DATA. iS THERE ANY WAY THE LIST IN THE DROP DOWN LIST CAN BE UPDATED FROM MANUAL ENTRY FIELD?

You can use the widget called User Contributed Dropdown. In the dropdown, there will be an option that states "Add Option".

You might also want to check the Configurable List widget. Here's a guide: How-to-Set-Up-the-Configurable-List-Widget

Hope that helps. Thank you.