majorly modified template based on open source template with free license on internet, ok to upload?

Hi sir,

can we build our template ( with major modifications ) on open source templates or purchased templates ( templates with free license on the internet, not on themeforest) ? would that be accepted upload that we can sell on themeforest? everyone needs to start from scratch and we are but one dont need to waste time to get the bricks yourself when you are designing a great building… any answer is appreciated.