Is it a list form or a form deployed in a forms library? If it's a form deployed to a forms library, check the data connection, maybe the author for the form missed checking that field and retrieving it in the data connection.
You can also open the lists directly and check in which list/library the field exists, if it's in another library, you can query that library through a data connection as well, and get that field's value through a condition depending on the current item's values. For example, if you have List1, and List2, and you're developing the form on List1, you can query List2 for your target field depending on a condition for List1 item that you are on, because when you query a list it will retrieve a collection of items, and you need a condition to get only one item, once you do that you can store it in your form and use it in a workflow.
If you need to only get the field in your workflow, without the need to use it in your form as you've stated in your update, then you can do so inside the workflow itself. In the workflow you can define a variable, and set this variable to do the value of a lookup, and choose the other list from the lookup dialog, then choose the field that you want, for example, in the lookup dialog you'll choose List1 as the data source, and choose column1 as the field. At the bottom you will see 2 sections, these represent the condition, to specify which item exactly that you need to lookup. Remember you are looking up a column1 of the list, but the list has many items and you need only 1 item to get. So at the bottom you specify the condition, for example, choose ID, and in the last field you'll enter the value of that ID OR you can get column1 for the item that has the ID of the current item.
It all happens in that lookup dialog, you should spend a bit of time checking it and you'll figure it out. When you get the data, you can easily set value of current item. Let me know if you need more help!