conditional formatting changes the format of the selected object. It will not change its value.
There are, however, several ways the action of changing the record's status can be used to change the value of your field.
Write a script:
If [ YourTable::Status = "Omit" ]
Set Field [ YourTable::YourField ; If ( YourTable::YourNumber > 0 ; YourTable::YourNumber * -1 ; YourTable::YourNumber ) ]
Use a script trigger on your status field to perform this script.
Depending on what you intend to do with this field, it might also be possible to use the following if function as an auto-entered calculation.
If ( self > 0 and Status = "omit" ; self * -1 ; self )
Clear the do not replace existing value check box if you use this calculation