I think the OP wanted to mix a seeded wallet with watch only addresses which would appear to be not possible in accordance with the FAQ (so he would need to create two separate wallets).

Now that Bitcoin supports "pruning" (with a wallet) perhaps the OP should consider using Bitcoin Core instead of Electrum (that should let you create a mixed wallet although you would lose the ability to restore from a seed so you'd have to import all the private keys into a new Bitcoin Core wallet and then be sure to create regular backups of that wallet).