How do I reheat previously-cooked frozen sausage?
One option: reheat slowly in the microwave and until it is barely unfrozen enough to break apart.  Then break it down into meal-sized portions, take the part you want for now and wrap the remaining bits in cellophane so they don't freeze together again.  (My advice as a self-judged microwave expert is to heat it up at low power in 3 minute increments at first, then 2 minute increments.  Fairly early, you will find that some parts thaw faster than others.  Once this temperature difference becomes apparent, you can wait like 5 minutes in between bouts of heating it up to allow the heat to even out over the lump.)

Given that the freezing and thawing will probably not do anything good for the texture, you might consider breaking the sausage up and adding it to something like sawmill gravy or strata (or just scrambled eggs) where you won't be eating it in large chunks.