I've found an older (not really compatible with the current libssh git master) patch about
the diffie-hellman group-exchange algorithm implementation in this e-mailing thread:
I decided to rework it a bit, of course it is based mostly on Yanis Kurganov's patch.
I am sure that this needs still some reworking to get in, but I am filing this ticket here as a start.
I've done some real-life testing of the client part on various distributions SSHd servers
(where only these KexAlgorithms were enabled), however I haven't checked the server part that deeply..
OFF: I've tried to send this to the mailing list, but apparently my mail haven't arrived there...
Please find the updated patch attached to this issue.