If the numbers are in a table, you could press Ctrl-F9 to create a field, type '=SUM(ABOVE)' between the field braces and press F9
to update the field. Note: not reliable if there are blank rows.
You can create formulas using Word but there are alot of limitations. You
might want to try "Formula Builder for MS Word." It has a free demo version
at most major software download sites.