If you meant pre-compile time then that could be done, but I'm not sure of the approach you are after since every Component in a 64-bit installer is typically marked as either 32-bit (default) or 64-bit. But, you can save an .ISM as "Project File Format" of "XML" instead of the default "Binary" file format and then use XML tools to write to or alter the .ISM file prior to compiling.
I typically have two .ISM files - one for 32-bit authoring (32-bit MSI for 32-bit or 64-bit systems) and one for 64-bit authoring (64-bit MSI for 64-bit systems).
A 32-bit installer cannot write to 64-bit locations on a 64-bit system.
One of our most complex products I wrote a 32-bit installer that was used n both 32-bit and 64-bit systems. Then if you had a 64-bit system it ran this overlay that placed the 64-bit registry keys and files only. It kept me from having to maintain two huge installers that were basically identical. Instead I maintain one huge installer and one overlay installer that was a lot smaller. It worked out very well.
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