I have no tried that, but it can probably be done.
First, however, I do not believe that a 32-bit UMAT in dll-format would on a computer with a 64-bit OS.
The approach I would try to accomplish what you are trying to do is as follows:
1) The Intel Fortran compiler typically installs both a 32-bit and 64-bit compiler, even if the host computer is only running 32-bit Windows.
2) Compiler your UMAT into object format using the 64-bit compiler
3) Link the dll file using a 64-bit linker and the appropriate Abaqus 64-bit libraries (which you can copy from your 64-bit computer)
4) Copy and use the 64-bit dll file on your 64-bit computer.
Note 1: It is not a trivial task to accomplish these tasks. It will take some experties to figure out the compile and linking commands.
Note 2: I basically use this approach to compile by 32-bit and 64-bit dll-libraries, but I have not tried to compile the code using a cross-platform compiler.