You can solve it by leaving the entry blank instead of using a star. By default * means all listed values, not all values.
Alternatively you can put this line above the section access section:
Star = '*';
That will change the default behaviour to make the * mean all values instead of all listed values.
I think that it's not SET Star='*', but STAR is '*' the correct syntax.
But according to QlikTech this instruction is obsolete (and it doesn't work with field used as key, so not sure it works with a section access table which is used to be linked to other tables).
Regarding the number of combinations, if you build your section access at the end of your script, you just have to concatenate the existing values of your datamodel.
In your example:
LOAD * inline [
ACCESS, USERID,REDUCTION, OMIT
USER, DOMAIN\B, 2,NUM
USER, DOMAIN\C, 3, ALPHA
Concatenate LOAD Distinct 'USER' as ACCESS,'FAKEUSER' as USERID, REDUCTION,NUM Resident T1 ;
But if it works fine with empty value, it's less work