You can't extend the date beyond 1792, but you can change settings so that you essentially create new scenarios. For example, you can make the United States have a rebellion earlier than 1770's and even set it up to exist at the start of the 1492 campaign (if you wanted.. though maybe not such a good idea). There are just a handful of files to edit to achieve these effects:
Scenario file = The Grand Campaign.eug (for example)
Scenario Settings file = 1492.inc
Master Settings File = text.csv
Editing them carefully is the problem. Make backups before you do. Also, get a hold of the IGC and look at some of the things that Doomie and Hartman changed so you can get the idea of what settings correspond with what fields and values.
