On the "Values" properties box of the combobox control, add a "Formula Item". The formula you write should return an array of strings. Every string in the array can have a label and value if you add a pipe (|) character to it, e.g.:
[
"label1|value1",
"label2|value2"
]
In the pre-XPages era, this syntax was also used to add values to a combobox on a Form.
Mark