I'd first question if your program really doesn't accept scientific notation, if so that creates problems for working with these values and you should consider using a properly implemented program. The following example shows what I mean: In principle, the following bash code does the conversion just fine:

while read -r line;
do
   printf "%.325f\n" $line ; ## change format to %.308f as minimum sensible representation 
done < testfile.txt

You need to decide for some sensible cutoff of precision, I am using 325 just to show the problem:

Input (your range):

1E-5
1E-100
1E-324

Output:

0.00001000000000000000081803053914031309545862313825637102127075195312500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000199918998026028836196477607885341594201826030059365956992555434676176762886132929895827460748109118507985282705397496540222684360419612636083562831412787179427249289424690806658916305930004345786023014502507945 

0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Note: 1E-324 is effectively zero in this notation but maybe smaller than the smallest positive non-zero double precision value. On 64bit platforms that is 2e-308 **

Of course you could do some trickery by having variable precision floating point numbers, but you rather don't want that.

** checked with the c program:

#include <float.h>
#include <stdio.h>
int main()
  {
    printf("%.e\n", DBL_MIN);
  }