Add Metadata to

Ok, so now you just need this to work on 2 or more open docs? There could be 2, 3 or more files open and this varies each time the script is run?

Edit: Here is an updated version that works with 2 or more open docs. There is no longer a requirement that the template is the first doc open, the active document when running the script should be the template, and the order doesn't matter. The pasted images are converted to embedded smart objects.

/*
Copy and paste pixel and keyword metadata.jsx
v1.3, Stephen Marsh, 18th October 2022
https://community.adobe.com/t5/photoshop-ecosystem-discussions/add-metadata-to/td-p/13196138
*/

#target photoshop

// Check that at least two docs are open (template + 1 file to combine)
if (documents.length >= 2) {

    // Set the template doc as the target
    var templateDoc = activeDocument;

    // Get the operator to confirm that the active doc is the template
    if (confirm("Add all open docs to the current doc?")) {

        // Loop over all open files
        for (var i = 0; i < app.documents.length; i++) {
            activeDocument = app.documents[i];
            copyPasteImageAndMeta()
        }
        
        // Remove any metabloat from the original template file
        // https://prepression.blogspot.com/2017/06/metadata-bloat-photoshopdocumentancestors.html
        if (ExternalObject.AdobeXMPScript === undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
        var xmp = new XMPMeta(activeDocument.xmpMetadata.rawData);
        xmp.deleteProperty(XMPConst.NS_PHOTOSHOP, "DocumentAncestors");
        app.activeDocument.xmpMetadata.rawData = xmp.serialize();
    }

} else {
    alert("There must be two or more docs open to use this script!")
}


function copyPasteImageAndMeta() {

    // Exclude the template doc from the loop
    if (activeDocument !== templateDoc) {

        // Copy the iptc keyword / dc:subject metadata
        // var keys = activeDocument.info.keywords;
        if (ExternalObject.AdobeXMPScript === undefined) ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
        var xmp = new XMPMeta(activeDocument.xmpMetadata.rawData);
        var keys = getArrayItems(XMPConst.NS_DC, 'subject');

        function getArrayItems(ns, prop) {
            var arrItem = [];
            var items = xmp.countArrayItems(ns, prop);
            for (var i = 1; i <= items; i++) {
                arrItem.push(xmp.getArrayItem(ns, prop, i));
            }
            return arrItem;
        }
        //$.writeln(keys);

        // Remove the file extension from the source doc name
        var docName = activeDocument.name.replace(/\.[^\.]+$/, '');
        // Copy the pixel data - single layer source
        if (activeDocument.layers.length === 1) {
            activeDocument.activeLayer.copy(false);
            // Copy the pixel data - multi layer source
        } else {
            activeDocument.selection.selectAll();
            activeDocument.activeLayer.copy(true);

        }
        //activeDocument.close(SaveOptions.DONOTSAVECHANGES);

        // Paste the pixel data
        //$.writeln(activeDocument.info.keywords);
        activeDocument = templateDoc;
        activeDocument.artLayers.add();
        activeDocument.paste();
        executeAction(stringIDToTypeID("newPlacedLayer"), undefined, DialogModes.NO);
        activeDocument.activeLayer.name = docName;

        // Append the keyword metadata
        // activeDocument.info.keywords = keys;
        // https://www.photoshopgurus.com/forum/threads/pull-layer-names-for-file-info-keywords.69966/#post-1533794558
        if (!ExternalObject.AdobeXMPScript) ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
        xmp = new XMPMeta(app.activeDocument.xmpMetadata.rawData);
        // Uncomment the line below  to clear the keywords field
        //xmp.deleteProperty(XMPConst.NS_DC,'subject');
        for (var s in keys) {
            xmp.appendArrayItem(XMPConst.NS_DC, "subject", keys[s], 0, XMPConst.PROP_IS_ARRAY);
        }
        app.activeDocument.xmpMetadata.rawData = xmp.serialize();
        //$.writeln(activeDocument.info.keywords);
    }
}