How many different strings do you want to convert back and forth? For a 5 digit number, the maximum possible set of strings that you could map back and forth is under a million. It's a pretty straightforward result of the Pigeonhole Principle.
So if you want to do this, you're going to have to come up with rules to constrain your set of input strings to a set small enough to fit in the number of pigeonholes you have. *AND* you'll have to come up with a unique mapping between those strings and the numbers. (You, not us.)
The instant you have the possibility of mapping two different strings into the same number, you have lost information, and can no longer tell the two strings apart from the number. There's no way to reliably know which string to map the number to without some other source of information.
Sorry for the troll food.