Hello. I just finished Black Mesa: Source but in Original HL1 there is ending on Xen. Will you add it in update please?
It’s already been confirmed that Xen will come later. Not this year, and with the Deathmatch Multiplayer as well. Be patient.
And what about SP COOP like Synergy for HL2 series? Or they should just attach it to the synergy.
They are adding it. Be patient.
check my post under half life series, there’s already a way to set that up from the console even though is crashes often 
You haven’t read anything about the mods development have you?
they have stopped making coop for black mesa
If you want a taste of Xen, there is a door in the chapter “Questionable Ethics” where it is labeled “Observation Area”.
Until then, they are working on Xen.
Question answered.