input the data from the file using the regular methods:
open file AS #1 for input
and us either input or line input
then set the row source type to value list
then you can add the fields to the box by setting row source to the values. example code would be:
dim box as combobox, temp as string
open file.txt AS #1 for input
while not eof(1)
temp = temp & input(1, 5) & ";"
wend
box.rowsource = temp