Seems that many people become scared as soon as they seen 1 week or over and discover they've to download the full blockchain(ca. 60 GB now required), so it's full of question about migrating from bitcoin core to electrum? But what about vice versa? Bitcoin core doesn't support sweeping, but it should support, by command line, importing private addresses. First questions, I read there is dumpwallet, is there a importwallet option that allow to import more than 1 private key at time? This could be a simply option, import all keys at time. Import each keys with importprivkey can be a long operation. Then, 2nd option, is just send electrum btc to a bitcoin core address right? What about get the electrum wallet, is bitcoin core able of read it in some ways (not with external software)? Is there a way to safely convert?
Coverting a wallet from Electrum to Bitcoin Core is very hard, as both pieces of software have a very different idea of what a wallet is. Electrum relies on servers that provide indexed blockchain information to discover transactions, so its wallet does not store much information. Bitcoin Core needs all your transactions and keys actually available in the wallet so that it can operate independently.
With Electrum's command line you can dump all your private keys, or only those that are funded. See http://docs.electrum.org/en/latest/cmdline.html#export-private-keys-and-sweep-coins
FWIW, someone created a tool. A cursory examination doesn't show any security issues: