Is there a way to use dropdown list that its options will be auto updated from an external source?

Is there a way to use drop down list that its options will be auto updated?
I would have client list in drop down list. But what should i do when list of clients will increase?
Is there a way to not update it manually?

You are currently using our User Contributed Dropdown widget. This widget can be useful if you want users to dynamically add an option in the dropdown. If you don't want this to happen and if you want to manually update it instead, you can use our basic dropdown field from the Basic Fields section.

To add a client, you will have to edit the form and list it down in the dropdown options. 

I used this list only for testing. I update a list of customers in the spreadsheet google and google contacts with additional applications. Is there any possibility that the value from the drop-down list have been taken from outside, for example with google spreadsheet?

We don't have such feature to populate a dropdown from an external source. You'll have to code that outside Jotform builder. You can get the form source codes and manipulate it to update the dropdown.

https://www.jotform.com/help/104-How-to-get-the-Full-Source-Code-of-your-Form

I cannot give you an example, I'm sorry for that. But I've found the following page that would help you achieve it.

http://stackoverflow.com/questions/21392445/update-dropdown-from-spreadsheet-in-apps-script 

Let us know if you need further help on this.