Since the RC1 version, conversion tab has been improved to become more user friendly. First of all, the box "Convert" must be checked in order to process conversion into the FITS Siril format. Then, click on the "+" button to add the files to be converted. All supported formats, except FITS, can be selected at same time and will be converted in a sequence of files starting with the generic name you've specified just below.
Name_XXX