In COMSOL it is possible to solve a problem for a set of modes in the absence of damping, and then to use those solutions as a modal basis to solve a problem in the time (using a time domain modal study) or frequency domain (using a Frequency Domain, Modal study). In both of these cases it is possible to manually assign a damping ratio to the computed modes in the time or frequency domain study. To do that, right click on the study and choose Show Default Solver, then expand the solver sequence until the Modal Solver node is visible. In the settings window for that node, add damping ratios for each of the modes.