you actually can. what you can't do is importing private keys into a wallet that was created with a seed. it is like this to prevent accidental loss since the seed will recover all the private keys apart from imported ones.

to import private keys just create a new wallet (Ctrl+N) select the first option (Standard Wallet) next and select the third option (Use Public or Private keys).
then enter a list of private keys or even a master private key (xpriv...) and use them.
for example OP can use  https://iancoleman.github.io/bip39/  to generate the xpriv and import that into electrum without needing to worry about seed differences.