Sorry about that Don, NEF files are a pain in the backside

Hopefully this code will work for NEF file only!

It update the xmp file so after it has run you would need to refreash the folder (or select a diffirent folder and go back)

#target bridge   
if( BridgeTalk.appName == "bridge" ) {  
combineFields2 = MenuElement.create("command", "NEF Details to Description", "at the end of Tools","cfta2012");
}
combineFields2.onSelect = function () { 
if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
var sels = app.document.selections;
for(var z in sels){
var Name = decodeURI(sels[z].spec.name).replace(/\.[^\.]+$/, ''); 
var file = File(sels[z].spec.path + "/" + Name +".xmp");
if(file.exists){
     file.open('r');
     file.encoding = "UTF8";
     file.lineFeed = "unix"; 
     file.open("r", "TEXT", "????");
     var xmpStr = file.read();
     file.close();
     }else{
         var xmpStr='';
         }
     var xmp = new XMPMeta( xmpStr );
var arrItem=[];
var items = xmp.countArrayItems(XMPConst.NS_DC, "description");
for(var i = 1;i <= items;i++){
 arrItem.push(xmp.getArrayItem(XMPConst.NS_DC, "description", i));
}
try{
var Desc = arrItem.toString();
var City = xmp.getProperty(XMPConst.NS_PHOTOSHOP, "City");
var Country = xmp.getProperty(XMPConst.NS_PHOTOSHOP, "Country");
var Region =  xmp.getProperty(XMPConst.NS_PHOTOSHOP,  "State");
var DescAll = Desc + "\r" +City + "\r" + Region +"\r" + Country;    
xmp.deleteProperty(XMPConst.NS_DC, "description"); 
xmp.setLocalizedText( XMPConst.NS_DC, "description", null, "x-default", DescAll ); 
file.open('w');
     file.encoding = "UTF8";
     file.lineFeed = "unix"; 
     file.write( xmp.serialize() );
     file.close();
     }catch(e){alert(e+"-"+e.line);}
    }
}

It might be best to use commas to separate the different fields Don.

This version appends each field with a comma and space between them ...

#target bridge   
if( BridgeTalk.appName == "bridge" ) {  
combineFields = MenuElement.create("command", "Details to Description", "at the end of Tools","cft2012");
}
combineFields.onSelect = function () { 
if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
var sels = app.document.selections;
for(var z in sels){
var t = new Thumbnail(sels[z]);    
var md = t.synchronousMetadata
var Desc = md.read("http://purl.org/dc/elements/1.1/",  "dc:description");
var City = md.read("http://ns.adobe.com/photoshop/1.0/",  "City");
var Country = md.read("http://ns.adobe.com/photoshop/1.0/",  "Country");
var Region =  md.read("http://ns.adobe.com/photoshop/1.0/",  "State");
var DescAll = Desc + ", " +City + ", " + Region +", " + Country;    
var myXmpFile = new XMPFile( t.spec.fsName, XMPConst.UNKNOWN, XMPConst.OPEN_FOR_UPDATE); 
var myXmp = myXmpFile.getXMP();
myXmp.deleteProperty(XMPConst.NS_DC, "description"); 
myXmp.setLocalizedText( XMPConst.NS_DC, "description", null, "x-default", DescAll ); 
if (myXmpFile.canPutXMP(myXmp)) { 
        myXmpFile.putXMP(myXmp);
         myXmpFile.closeFile(XMPConst.CLOSE_UPDATE_SAFELY); 
        }
    }
}

The line that you might want to alter is ..

var DescAll = Desc + ", " +City + ", " + Region +", " + Country;  
 

If you don't want the commas you could remove them just leaving a space.