The i386 Debian package file should be able to be "forcefully" installed on an amd64 system via
dpkg -i --force-architecture
This will probably leave other issues lingering before the application can be used; the package probably depends on other libraries, specifically the i386 versions of these.
This is addressed in Debian, and therefore Ubuntu, by the multiarch project. This enables a system to have the same libraries compiled for different architectures installed simultaneously. However, perhaps the package you are referring to does not have any problematic external package dependencies, and then you won't need these multiarch libraries (the forum post linked below kind of suggests this is the case).
How to continue from this point depends on the packages internal dependencies and layout. This Ubuntu forum post seems to handle the exact situation of installing Retrospect on 64-bit Ubuntu, opting for
alien conversion from RPM to Debian package format
- manually changing the
i386 string to
amd64 in the
- rebuilding the package.
I propose that step 2 should not be needed with the
--force-architecture switch to
dpkg, but if you do some experimenting, you will probably see how it works in practice.
Note that the forum post lists the packages
libc6-i386 as prerequisites for Retrospect (this is where multiarch might be needed, but the person in the forum post seems to have managed without).