Unfortunately this is not possible on the DF1000.  It is literally just a plug and play device

The only option on the unit is to clear the 32 automatically set filters manually and let it start watching again. It does recycle the filters when it reaches the 32nd filter, but it just removes the first one and replaces that with a new cut, and so on.

However, if this feature is required, then by using a DM8000 / DM8500 you can set up a feedback suppressor with up to 16 bands, with widths of your choosing and choice of fixed or floating granularity.  Plus, you can stick multiple feedback suppressors into the design.