Hi Mariana,
No function to import a time-frequency file from fieldtrip, but the possibility to compute directly the time-frequency decomposition using FieldTrip’s multitaper functions. Process Frequency > FieldTrip: ft_mtmconvol.
If your goal is to review in Brainstorm files computed with FieldTrip, a simple solution is to compute the equivalent file in Brainstorm, and replace the data matrix with your own results:
http://neuroimage.usc.edu/brainstorm/Tutorials/Scripting#Custom_processing
Cheers,
Francois