Can a hero join a group of henchmen, if so how does it affect their pool of abilities?

I don't think so.

The henchmen rules as written (GM Guide p 49) are trying to track a large number of equivalent NPCs as one group of stats.

And while it doesn't expressly say all the henchmen must have completely identical stats, the henchmen rules ( don't act on your own; die when wounds = toughness) would seem to make it a bad choice for a hero.

I look at the henchmen rules as GM shorthand for the waves of minions found in action movies everywhere. The heroes carve a bloody path through them on the way to confront the big boss. Collective actions (and the fact they scale to party size - something I missed in my example) makes the weaker NPCs still able to matter, without overwhelming the party.

Adding a hero/nemesis to a group of henchmen is putting a boss among the minions exposes the boss to more harm and undermines the primary purpose of henchmen (serving as a buffer to the boss).

You certainly could have a hero work right next to a group of henchmen and they could work in tandem (aid, confer ally bonuses) but actually merging their stats into a henchmen pool runs counter to how I interpret the rules.