I would like to import "files1.dat" (there is a hundred of files..). Then I would like to process them with a band-pass filter (for example from 10 GHz to 11 GHz). And then, export them in ASCII.
Origin supports several kinds of FFT filters, including band-pass, please refer the X-Function, fft_filters.
I suppose all the files are similar and have same number of columns, then you can use Batch Processing to perform FFT filter for all files at one time.
Please refer these relative tutorial page if needed.