The answer to your question is yes.
Current NHL teams are allowed to negotiate with the Vegas team, although it is completely up to Vegas to accept the offers. It is common in expansion drafts (in most sports) to have existing teams convince the new team to not select a player by giving them a certain package in return.
The trade would officially be as follows. The existing team sends a player/pick package in exchange for "future considerations", which in this case, refer to the expansion team not selecting (or selecting) a certain player.
For a comparable, look in the 1998 NHL expansion draft, with the Nashville Predators:
Los Angeles traded Kimmo Timonen and Jan Vopat to Nashville after the Predators agreed not to select Garry Galley.
This is one of a many deals that have been made over the years regarding the expansion draft process.