Hi, and welcome:
If you add a column to your secondary data source that you'd be able to bind your field to, then walk through the data connection wizard again, your column will be available. So, for example, if your secondary data source was from SQL and was a table called Customer, and you wanted an 'Active' checkbox, you could add a column to your Customer table of datatype bit and then in InfoPath walk through the data connection wizard again to select that new column, you'd be able to put a checkbox on the form that displays the value of the 'Active' column.
Beware -- a lot of times new users get confused with main and secondary data connections. InfoPath will only store data in the main data source. So any changes made to your secondary data connection bound controls are not saved (unless you specifically write code or find another route to update that secondary data connection). In other words, going back to my initial example, if you had that 'Active' check box on your form, and selected and deselected some, that would not be reflected in your Customer table, and the next time you open the form, you'd see all the Customer data in the same state as it is in your database. This blog post by S.Y.M Wong-A-Ton may help you understand a bit more if I've not been clear enough.