ans. You can freeze casters.
Better alternative is to vacuum pak them as suggested, if you want them reasonably fresh..(ish). I would first put your leftover casters into a bucket of water and rinse off the floaters and drain and dry the sinkers on paper towelling before placing in a vac tub. That way you maximise the quality of them especially if you choose to catch roach.
Freezing them in water and the result of thawing is for black smelly water and casters that are quick to turn black, which although to you and I is near puke producing, bream seem to like it and the pigs with fins eat almost anything at this time of year.