When using the native FITS encoding, multiple Objects may be stored and all I/O operations are sequential. This means that you can simply write a sequence of Objects to a FitsChanFitsChan. After each write operation, the CardCard attribute will be updated so that the next write appends the next ObjectObject description to the previous one.
If you then rewind the FitsChan, you can read the Objects back in the original order. Reading them back will, of course, remove their descriptions from the FitsChan (ยง16.10) but the behaviour of the Card attribute is such that successive reads will simply return each Object in sequence.
The only thing that may require care, given that a FitsChan can always be addressed randomly by setting its Card attribute, is to avoid writing one Object on top of another. For obvious reasons, the Object descriptions in a FitsChan must remain separate if they are to make sense when read back.