You can convert this to a long, do your arithmetic, and convert it back to an ASCII-Decimal number:
Remove the decimal point from the string. Convert the remaining string of digits to a long. Now you have the coordinate in binary, in units of the least significant decimal digit; 1 lsb == 0.00001, if you used 5 decimal places.
Manipulate your data however you need to using long integer arithmetic.
Divide the value by 10^ -(# of fraction digits you used). The quotient is the whole number part; the remainder is the fractional part. Print the quotient and a decimal point. Print the remainder padding with zeros to the left as necessary to result in your printing (# of fraction digits you used) total fraction digits.
The output conversion is even easier if you're willing to use sprintf():
Divide the value by 10^ -(# of fraction digits you used), as above. Let's assume 5 places as in your example:
char buffer[BIGENOUGH+1];
sprintf(buffer, "%s.%05s", value/10000, value%10000);
Note that I made two divisions to get the quotient and the remainder and assumed you could tolerate the performance hit. If you're expecting to crank out a lot of values and speed is a factor, you could look up or write a division routine that can return both the quotient and the remainder results from a single division.