Well you wouldn't do that in FileMaker--as there is a much better way to set that up.
The key word in your post is "Table". What you do is go to manage | Database | tables and define a new table for storing this data where each column in your table example corresponds to a field defined in this new table. Then you link this new table to your existing table in a relationship. This may require you to define an auto-entered serial number field in your original table and update existing records with serial numbers (see Replace Field Contents in the Records menu) before you can set up that relationship to work.
THen you would add a portal to your layout to get what looks and functions like your "Table in a field".
I do not think that I made myself very clear here.
For Example, Say I have a record that is eventally going to be containing cumulative data not being sourced from anywhere in the database.
I want to be able to record entries of this data transaction by transaction as it happens, so that when it is time to complete a record (such as month end), I already have a running total available to me to plunk in. Keep in mind, this data isn't relating to anything else already recorded in the database.
An easy example of this is that I am tracking mileage on a trip by trip basis. So each time, I want to input the mileage, so at the end of the month, I have a running total that I can have automatically transferred into the record.
That does not change my suggestion to use a portal, though I will point out that you do not need a separate record for the total if you have the individual values entered into separate records. There are ways to show the group of records entered--possibly in a portal and then show the computed totals for that group of record. This could be done with a portal or with a list view and the use of summary fields.
This can be done very easily. But I think you are not really having a Filemaker mindset yet. I understand that you are new and it might take a while to see things differently.
I think you can do two things: Either make a very simple list view that uses a subsummary part to calculate your totals. For instance, if you know already that you will want a total per month you create a subsummary total on a month field that can be calculated from the date.
If you don't really know when you will need these totals you can use two related tables. One where you define a name, date or month name or number or whatever you need to name or identify this one collection of 'milages' and then you enter your actual dates, times and amounts in the related table.
The totals can be calculated immediatly and can be displayed on your first table.
These are both really simple to make. If one of them sounds appealing let me know and I'll take you trough how to make it.