Is there a way to to export/copy wallet's XPUB from MultiBit HD wallet (windows)?
Not in the UI no.
You can work out the XPUB from your wallet words by making an offline copy of https://dcpos.github.io/bip39/ and then using that.
If it is a regular MultBit HD wallet it is the BIP 32 panel you want.
Use it offline for security of your wallet words obviously.