You need to design your database differently. Instead of having multiple fields for the different items, you only have one field, and then another item number field.
So, if a person has two items, you would have two records, i.e.
Code:
Name Item Num Item
Bob 1 Salad
Bob 2 Salmon
...
So your button would create a new record, not a new field.
Note that this is a "Normalized" database design, which should always be your goal. Normalized databases are much easier to work with.
For example, let's say that you had ten item fields. And someone asked you to find all the people who ordered Salmon. You would need to search 10 different fields, which is ugly and a lot of unnecessary work. As opposed to just having to search one field.
Here is a link on Normalization:
https://support.microsoft.com/en-us/kb/283878
There are many others out there on the web.