Short asnwer is yes, PLBVFU1 can be used to model frequency ramps.
The PSSe manual 'MODELS.pdf', Generator models section, describes the parameters of the model. For FR testing, if you have the corresponding V values at each F value, use those V values, else, I did use 1.0 pu for all values of the V column with no simulation complains. I prefer to have the values for F in the 60 Hz range, not in p.u. You can build the PLB file in excel first, and get a graphic view of the ramp for F and then save it in CSV format, with extension .plb
Also check the post "Example using PLBVFU1 playback generator" for additional info.