I only know about Shimano chainrings so your possibilities may vary. If you have a normal Shimano double road crankset on a bike, the spider's bolt hole dimensions are usually 130mm. The problem is that the smallest chainring that you can put on a 130mm crankset is a 39t chainring (I vaguely remember hearing about a 38 but have never seen one). So you would end up with at best a large chainring of your choice such as a 52t and the smallest chainring which would be a 39t which is a far cry from a real compact.

Compact Shimano doubles use 110mm chainring bolt spacing so they can use much smaller inner chainrings and end up being a real compact.

If your Campy spider can handle small chainrings, then you could swap out the small chainring for a smaller one of your choice and that should work provided the front derailleur can handle the larger spread between the teeth counts.