If the values can actually be converted to integers then you can use the Z format to convert it back with leading zeros. So use INPUT() to convert to a number and PUT() to convert back to string.
ctfips = put(input(ctfips,11.),Z11.);
Now if the values either contain non-digit characters, or there are more than 15 characters then you will want to use character operations instead. Such as moving the spaces to the front and replacing them with zeros.
ctfips = translate(right(substr(ctfips,1,11)),'0',' ');
Or prefixing zeros.
if length(ctfips) < 11 then ctfips = repeat('0',11-length(ctfips)-1)||ctfips;
Or
ctfips = reverse(substr(reverse(cats(repeat('0',11-1),ctfips)),1,11));
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