Nordhackstedt is in what is today northern Germany, about 10km south of the Danish border. https://maps.google.com/maps?mid=1349827010
Prior to the 1860's this area of southern Schleswig was mostly Danish property, though always up for grabs with Germany.
Try the online Danish census database and see whether you can find entries for your Johannsens. http://www.ddd.dda.dk/kiplink_en.htm
or go directly to autocheck of databasehttp://www.ddd.dda.dk/dddform_uk.asp
Select "Sydslesvig" from County list
Select "Nordhackstedt" from Parish list
Enter "Johannsen" in the Name box
and click Search
The resulting list of names is organized alphabetically by the person's given name, so scroll down to the "Johann" entries. There is a "Johann P Johannsen" listed for the 1845 census with an age of 40. Click the "show household" button and you will find other names that seem to match the one you are interested in.
This online database has been of great help to me tracking down some of my Nielsen relations in that part of Germany. In fact, there was a Hans Adolph Johannsen (b. 1765 in Rabenholz) who married Anna Catharina Nielsen (b. 1762 in Rabenholz), my 4th great grand aunt. Maybe there are some family connections with your Johannsens?