Unitarity conditions and constraints in HistFactory

Dear Experts,

we have a case of a simultaneous fit of partial decay mode amplitudes.
Each of the partial decay mode is implemented as NormFactors and for the moment they are independent NF.
But in reality we have external constraints from other measurements that we can exploit, because we now from external measurements the total decay mode amplitude (i.e. the value of the sum of the NF that we want to measure).

How we can express the constraint on the sum of the partial decay mode amplitudes in the combined fit of the partial amplitudes in HistFactory?



If this is a simple constraints of the variable, maybe a redefinition of the parameters will work. Otherwise non trivial constraints are not supported by Minuit and this is not in theory possible. Maybe adding a new penalty term on the sum will work, but it needs to add externally and is not supported in the HistFactiory