I think you have non-numeric columns so you can't use a matrix, You might make your string value as a concatenation of items in a list, and you can get a list from a character column with the <<getvalues message.
alist = dt:a<<getvalues; show(nitems(alist));
astring = concatItems(alist,","); show(length(astring),left(astring,100));
N Items(alist) = 21000000;
Length(astring) = 207635028;
Left(astring, 100) = "1,4,9,16,25,36,49,64,81,100,121,144,169,196,225,256,289,324,361,400,441,484,529,576,625,676,729,784,";
JMP has a limit of 2 billion characters for a string. You might be getting close.
Craige