I looked briefly at the 1819 census for Mecklenburg in Ancestry........and
there are a lot of Borchardt's. You could use that list of towns as a
guide for checking out the availability of films of the church books at the
LDS libraries. THeir library catalog is online to make the search easier.
You can do the same for the 1860 Mecklenburg census, using Joachim's DOB as
a guide........looking for siblings that possibly didn't immigrate. But
that would tell you the main places where you can find that family name.
Check the WI genweb site to see if the naturalization records are housed by
state or county.
Do you know what year he immigrated?
Does WI Genweb have anything on Waukesha county that describes early
settlers?
If he owned land, there might be something that would zero in on date of
immigration (was there any type of residency requirement back then, or could
people who were not naturalized buy/own land?)
If you can determine the range of time for immigration, it will help.
Linda