If you have all your cards concatenated together into a single long string, each occupying 80 characters (with no delimiters), you can insert them into a FitsChanFitsChan in a single call using astPutCardsastPutCards. This call first empties the supplied FitsChan of any existing cards, then inserts the new cards, and finally rewinds the FitsChan so that a subsequent call to astReadastRead will start reading from the first supplied card. The astPutCards function uses astPutFitsastPutFits internally to interpret and store each individual card, and so the caveats in ยง16.8 should be read.
For instance, if you are using the CFITSIO library for access to FITS files, you can use the CFITSIO fits_hdr2str function to obtain a string suitable for passing to astPutCards: