Is there a hack that allows us to transfer an existing Windows 7 installation to a Boot Camp partition on a Mac?
If there is, does it have any side effects, or shortcomings like being less efficient, etc.?
What sort of computer is Windows installed on? There will almost certainly be driver issues, but they may be overcome-able. A bigger problem will be the secret sauce Apple uses to make Boot Camp work.
Possible solution: install a fresh Windows 7 of a matching flavor (32 vs. 64 bit), then overwrite its partition with your existing install. This way you can use a more-typical Windows-migration solution, one that someone who knows more about Windows than I can supply.