No, this is not possible. The reason why is quite broad, maybe even too broad for SuperUser, given that you would need to explain the fundamentals on how x86 and x64 were created, and how this influences programming in general.
But to explain briefly, it comes down to this:
In the past, we had 16-bit processors. Then intel made the first 32 bit processor, also known as x86. This is the 8086, 80286 (286 for short) etc... This was basically a modifcation to the 16 bit processors, with extra instruction sets added. With every new release of their processor series, Intel added more instruction sets to the processor family, which in the end caused the instructionset to contain many many instructions. Intel could not just remove old instruction sets because that would mean there was no backwards compatibility, and Intel wanted to keep supporting older processors.
As the processor is 32 bits, there is an upper limit, namely the highest 32-bit number. This means, that memory assignment only goes up to about 3,5GB.
Back in the days, computers were not so powerful, so if intel would aim for 64 bits from the start, it would mean that much more time was spent calculating the same numbers so performance would degrade simply because the numbers to calculate with are bigger.
Besides 32 bit processors worked quite well for a long time.
At some point, AMD entered the market and introduced the 64bit processor. AMD created their own instruction sets to allow working with 64 bits while keeping the intel instruction sets for 32 bits in tact to allow for backwards compatibility with 32 bits.
Given that they are in fact different instruction sets, a programmer who creates 32 bit programs will call different routines than when they create 64 bit programs.
Now that I explain why this is hard to do, lets continue to explain the problem from a programming perspective.
When you code a program, you write your code first. If your code is backwards compatible for 32 bits programs, you simply cannot use 64-bits numbers nor can you address more than 3,5gigs of memory at the same time. Basically, you cannot cross any limits that 32 bit programs face or your program will crash when done so.
As you now have code, only you can actually run your program. In order to make it so that other people can run your program, you have to compile the code into an executable. This means that the easy to read code is converted into instructions that the processor understands. During compile, you specify if your program is meant to run on x86 or x64, and the compiler will generate code using the instruction sets based on that processor architecture.
As you can see, you can't just modify an executable and make it work on a different executable. You first need to decompile the program to code, then recompile it using different instructions.
That said, given that a program that is compiled for x86 will natively work on x64, it is safe to assume that a programmer creates an x64 program because he is going to break limits of the x86 architecture. So even if you were to turn the x64 version into an x86 version, it is likely that the program will be unstable as you will cross the limitations of what a 32-bit program can do.