Electrum does indeed use it's own mnemonic scheme that is pretty much incompatible with everything
However, Electrum will allow you to import a BIP39 Seed (if you select it from the Options during seed input)... But it will only generate the correct addresses if the wallet you are importing from uses a m/44'/0'/0'/0 type (aka BIP44) derivation path (like Mycelium). This means that Electrum will happily accept the MultiBit HD seed if you try and restore... but the addresses and keys will be different because MultiBit HD uses m/0'/0
The only wallet I know of that will properly import MultiBit HD seed (now that Hive is no longer operational) is Breadwallet... which is only on Android or iOS.
To switch from MultiBit HD to Electrum, just do what Kolloh originally suggested, create a new Electrum wallet and then send all your coins from MultiBit HD to Electrum.