Unless they built it in in later editions (I'm on 8) it isn't an option.
The data is stored as return separated entries, so you could set the "select all" button you create to enter these entries and achieve your goal
Say your checkboxes are: Red, Blue, Yellow, Black
Have your select all button perform the single step script:
SetField[ YourField ; "Red" & ¶ & "Blue" & ¶ & "Yellow" & ¶ & "Black" ]
Ninja's method works well, but here's a more flexible approach:
Set Field [ Yourtable::Yourfield ; ValueListItems ( Get ( filename ) ; "YourValueListName" ) ]
With that approach, you don't have to modify your script if you later update the list of values in your value list.
Good deal. Thanks Phil.