Filemaker won't allow you to do that and you don't really want to do that anyway.
Instead of adding additional fields, you should add more records in a related table of composers. These composers can be listed in a portal. Adding a composer can then be as simple as entering/selecting a composer on the bottom blank row of your portal.
To learn more about portals, look it up in filemaker's help--portals are a major problem solver in filemaker database--so you'll want to know all you can about how to use them.
I guess my main questions is is there a way i would be able to have it to add one more row on the portal if i needed it because i don't want a box with 6 rows of which are 5 are empty because i only have one composer on that job... thats why the "+" feature would be perfect...
A portal will have a fixed size in browse mode.
You can set it to slide up when it is printed or previewed to eliminate the space taken up by unused rows.
You can change this height in layout mode by modifying the height of an individual row or by specifying a different number of rows to be displayed in the portal.
You can't change this dynamically, however--though you can specify just 3 rows and enable the scroll bar to make more rows accessible when you have more than three rows in your portal.
Note: portals are most useful for data-entry. For printing, there are other layout designs that work more flexibly.