In FEMAP you can define the following damping:
The relation between the above values is the following:
The typical input in modal dynamic response is to define a constant modal damping in the full frequency range, this is simply done in FEMAP using the function type 7.. critical damping vs. frequency. For instance, the following picture defines a critical damping of 5% in the frequency range between 0 Hz and 2100 Hz. The results is the same if you define the frequency range between 0 Hz and 1 Hz because NX NASTRAN extrapolate the curve to the infinite.
But if you know the specific critical damping value to each frequency then you can enter using the above method, usually the variation of tis data is unknow.
From modal analysis results, you can't really deduct damping. If you had test data from a sine sweep for instance, you can figure out modal damping, but not simply from a modal analysis.
However, there are some tables of recomended critical damping ratios (e.g. 5% for bolted joints).