No, I did not find a broker to move funds from US account to Canadian based brokerage accounts. I am not even sure how will this work, especially considering that USD to CAD conversion would apply. The best way might be to sell your stocks/funds and invest that hard cash in Canadian based brokerage account, and buy US based ETFs through that.
"How can you maintain job in US being in Canada?" --> This is a very broad question. Do you have any specifics related to this they you'd like me to answer?
some Canadian brokerages like Questrade, Interactive Brokers etc allow you keep funds in USD itself (funding through USD CAD account) and without needing to covert.