You can just use the audio of Ben saying go to the Dagobah system over another shot like how it is used numerous times in the final act of ANH. You just need the right shot of Luke. Maybe something as simple as over a shot of Luke in the cockpit and then he instantly tells R2 he's going off to Dagobah.

Either that or dream sequence.

Maybe he has a vision or hears the voice while he's staring into space on board the falcon wrapped in the blanket post Ben's death.

Or could footage of Luke in the cot from the end of ESB be used as if he's sleeping on board the Falcon/Hoth and tricked into footage of him waking up from a fevered dream where Ben tells him about Yoda.

Or could the original footage be used but cut in straight after Luke drops into the Snow after blowing up the AT-AT.