No. You can't import an already existent wallet/private-key to Ledger.
That's not strictly correct... Mycelium uses BIP39/BIP44 compatible seeds... so theoretically you CAN import a Mycelium seed phrase into a Ledger. Read here: https://ledger.zendesk.com/hc/en-us/articles/115005165309-How-to-import-recover-a-backup-on-a-Nano-S-... This would also defeats the whole purpose of having a hardware wallet.
Whether you *should* import a seed that was not created on the Ledger is a different topic for debate... common sense would suggest that you shouldn't because, as TryNinja pointed out, it defeats the purpose of the hardware wallet (ie. seed/private keys never exposed outside of the wallet).If you want to give it a go for practice, have at it... but personally, I would not continue to use the Ledger on a long term basis with an "exposed" seed.
