Although it seems to be too late now - I suggest the following solution:
At a supervised classification you derive the signatures from training-areas which you have digitized as AOI-polygons (area of interest) directly into the satellite-image.
With the signature-editor you can get a corresponding signature for each AOI you previously set. When all signatures are ready (e.g. 1 Sig. for pastureland, 1 for woodland, 1 for water...) you can save them into a .sig-file (File-menu of the signature-editor > save). This .sig-file can be used as input for a supervised classification.
Regards