Armstrong, who previously worked with the Bees as fitness coach almost 20 years ago, is returning to the club as assistant to Rossi Eames.
He has just finished his stint as assistant coach of the New Zealand national side at the Confederations Cup in Russia.
Eames said: “Alex is in on Thursday – he’s just finished the Confederations Cup with New Zealand and I’m sure he’s got lots of stories to tell us.
“He’s chomping at the bit and he’s really looking forward to coming in.”
The Bees squad began pre-season training with a double session on Monday and will continue to build up fitness ahead of their first friendly of the summer, at Maidstone United on Saturday 8th July.
“Everyone started fresh on day one and they all came back in good shape,” added the Barnet boss. “It was tough and that’ll increase gradually through the next few weeks.
“It’s about getting technical and tactical understanding and getting mentally prepared for the 10 months we have in front of us. We know it’s not easy and it’s important to be in the right mindset.”
For Rossi’s full interview, see below: