You cannot change the "ASA" of film. The ASA is a rating applied by the manufacturer based on industry-standard testing procedures. It is what it is.
You can reduce the EI (exposure index) of film. I routinely rate my film at about half the ASA assigned by the manufacturer. I do that by means of testing under realistic exposure conditions (rather than the idealistic conditions that are the basis for the standard ASA tests).
It is also possible to reduce the EI to less than what would result from practical testing. However - there is a limit. As you reduce the effective EI, you must also compensate with a change in development. A reduction in EI always means a corresponding reduction in development. The problem that you will encounter is that you don't want the development time to be shorter than 4-5 minutes - anything shorter is likely to lead to uneven development. You can also compensate by diluting the developer beyond the manufacturer's recommendations, but you eventually reach the point where the diluted developer doesn't contain enough active ingredients to develop the quantity of film you are working with.
So if you are working with 'off the shelf' films of ordinary ratings, you will need to use a neutral density filter to force your exposure time longer.