The Green Party of California (GPCA) does not endorse presidential candidates. It holds a primary election, which informs GPCA delegates to the Green Party of the United States (GPUS) Presidential Nominating Convention.
At the GPUS Presidential Nomination Convention, candidates are nominated by this process. The GPUS will either nominate its own candidate, or will choose to have no nominee.
As Sanders will appear on Democratic Party primary ballots, he will not be simultaneously appearing on Green Party primary ballots, and hence will not receive delegates to the GPCA presidential convention from Green Parties that hold presidential primaries.
In states where the state Green Party holds a convention or caucus (because they don’t have the legal ballot status that would compel their state governments to conduct a Green Party primary), it is possible that some state Green Parties might designate one or more of their delegates to support Sanders at the GPUS convention.
However, at the convention, GPUS Convention Rules state:
5-10.5(a) No candidate shall be eligible to receive votes who has failed to declare in writing to the Election Administrator their intention to accept the nomination if offered and to cooperate with each member state party of the Green Party of the United States to have their name placed on each ballot line to which a member State Party may be eligible.
Given that Sanders will be appearing on Democratic Party ballots and is seeking their nomination, he could not simultaneously tell the GPUS that he would accept state Green Party ballot lines.
The Democratic Party convention will be held July 25–28, 2016. The Green Party convention will (likely) be held August 4-7, 2016. Let’s assume that Sanders does not win the Democratic nomination that weekend. It is theoretically possible that between the Democratic and Green Party conventions, Sanders could decide that he wants to be on the Green Party ballot and communicate that to the party. However given that Sanders has said he would be supporting the Democratic nominee if he did not win the nomination himself, this scenario is highly unlikely.
But to play it out, if Sanders did declare that he wanted the Green Party nomination, it would be hard for him to win at that point, because most delegates would be pledged to other candidates. Under our convention rules, delegates are bound on the first round to vote for a designated candidate:
5-10.2 During the first round of voting, all delegates except those instructed by their state party as "Uncommitted", "NOTA" or "No Nominee", must vote for their designated candidate, as instructed by their state party. In subsequent rounds, delegates shall vote as instructed by their state party.
Given this, Sanders would likely not be able to win the convention in the first round, and if another candidate had a majority of delegates on the first round, the nomination would be won by that person.
If however, no candidate won a majority on the first round, at that point delegates are free to cast their votes for any candidate still in the running at that point. Under that theoretical scenario, Sanders could win on a second or later round.
Finally one could also envision another theoretical scenario in which Sanders falls way behind in the primaries and drops out early, and then seeks the Green Party nomination - also highly unlikely. However if he were to do that, he would likely have missed being on the ballot in states (like California) that have primary elections, meaning he would miss out on a lot of delegates support by starting too late.
So in conclusion, there is no likely scenario other than Sanders completely abandoning seeking the Democratic nominationt now and starting to participate in the Green Party nomination process, for Sanders to be the Green Party nominee. Concurrently, the Green Party has a nomination process under which it will either nominate its own candidate, or have no candidate. There is no process under GPUS rules for the party to endorse someone else.
References: Rules and Procedures of the Green Party of California
ARTICLE I Presidential Primary Election Ballot
1-3 Written Requests
ARTICLE II Policies and Procedures for Selection and Conduct of GPCA Delegates to the GPUS Presidential Nominating Convention
2-1 Delegate Selection Committee
2-2 Delegate Allotments
2-3 Delegate Slates
2-4 Delegate Selection
2-5 Delegation Voting Rules
2-6 Ascertaining Ranked Preferences