Pajolatax: Was this feature removed? I remember playing it many years ago and there was option to fuse cubes and tactical cubes and make one big cube, it was big fun to do it and obliterate enemy. Now I am at the end of Borg campaign and this option is not here.
As I haven't seen the correct answer to this question:
The Fusion Cubes are only available in the last Borg mission. It was also only in this mission in the oribinal game. Although you could have used it outside the single player missions in skirmishes and PvP maps as far as I remember.
And you need to build a fusion hub as well, which is part of the technology assimilator, where you also can extract assimilated technology by dismanteling a cube, that assimilated technology.