In VBA you would use something like
ComboBox1.ListFillRange = "'[Excel-A.xls]Sheet1'!$A$1:$A$10"
replacing
$A$1:$A$10
with an appropriate range and
Sheet1
with the name of the worksheet containing the data you need. Note that ListFillRange can also be set on the properties for the control
The spreadsheet will need to be open ...
Application.Workbooks.Open ("c:\Excel-A.xls")