I'm just curious, but why on earth would you want to do this?
The biggest problem that you run into is that before the mid 1700's, you had to get into the where as well as the when for a date to have meaning... The Julian to Gregorian shift was not observed at the same time everywhere. See one of the shift explanations for more details.
In other words, May 1, 1540 was observed over an almost two week period throughout Europe. Without knowing where it was May 1, you don't really know when it was May 1!
By the end of 1763, most of Europe worked from one calendar. The Europeans chronicled nearly everything in the world, or at least offered an easy way to convert from other calendars to theirs. That was the first time in recorded history that a single calendar could be effectively used world-wide.
The short answer is that to store dates before what SQL Server supports, you really need to use a VARCHAR variable large enough to describe the date, since you need to store both location and that location's observed date. This gets really messy, and lots of tools (like Powerbuilder) just ignore the difference, figuring that any dates before about 1805 are probably estimates anyway.