Victoria,
Are you talking about individual Democrats, or officially affiliated Democratic Party organizations, like clubs and Central Committees? Individual Dems can and do endorse at will. Clubs and Central Committees have bylaws prohibiting it, but you'll see Dems ignore their own bylaws and make exceptions when they need to do so in order to remain relevant to the electoral process. You'll never get a Dem Central Committee endorsing a non-Dem, but the clubs will, if the candidate is compelling enough that they can run a credible campaign even without them. Especially the progressive clubs that see Greens as leverage against more conservative elements within their own party. I saw this happen at one point in Santa Cruz County.