yes you can by a couple of ways if you know how to edit giant's code system and paint.net for dds files.
1. make a back up copy of the map file in case you screw something up.
a. unzip the map file. Look in the files for a folder probably labeled as objects. you will have to dig for it.
b. open the dds file using paint.net and cover the existing name with the same background color so to wash it out.
c. add your name to the same area.
d. re-save the file without changing the file name.
e. re zip the map file and paste back in your map folder.
check your map. you should see the sign in your name. remember, i think there is 2 sides to that sign so there might be 2 areas to change.
this process is for using a sign mod from a mod site.
2. Do step #1 (requires knowledge of the Giant's editor)
a. make 2 extra folders and do step #1a in one folder and leave the map zipped in the other.
b. in the unzipped folder - open the dds to be sure that is the sign. Take note of the filename.
c. in the other folder - open the zipped map file using Giant's editor.
d. look for the sign in the transform groups. and just delete the object.
e. Save the map file in Giant's editor.
f. find a signage mod from any mod site that appeals to you and put the mod in your map/mods folder.
g. be sure your map is re zipped and place back in your map/mods folder.
h. go to the place where the johnson farm sign used to be and purchase your new sign and place it.
The easiest method.
3. Do step 2f.
a. purchase a sign and place in front of the existing sign to block it. (may need to do the other side if it has the name also.)