This is a little more complex, but it can be done. The easiest way (that I’ve found, anyway, is to copy a “Set Named Clipboard to text” action as XML, paste the XML in the AppleScript, and replace the part of the XML containing the string with "& VariableName &" like this:
tell application "Keyboard Maestro Engine"
set clipText to "Test"
do script "<dict>
<key>JustDisplay</key>
<false/>
<key>MacroActionType</key>
<string>SetClipboardToText</string>
<key>TargetNamedClipboardRedundantDisplayName</key>
<string>Sig</string>
<key>TargetNamedClipboardUID</key>
<string>15F896EC-ABA6-476A-BA07-4FA02B9B71B7</string>
<key>TargetUseNamedClipboard</key>
<true/>
<key>Text</key>
<string>" & clipText & "</string>
</dict>
</array>
</plist>
"
end tell
I’m afraid you’ll need to replicate this with your own clipboards, since this one is unique to my system, but the principle should be the same.