Centre-forward Johnson has been snapped up on a six-month loan from League Two rivals Mansfield Town.
He made an instant impression by netting in the Saddlers' final pre-season friendly, a 2-0 victory at Solihull Moors, and Flynn hopes to follow up the 29-year-old's arrival with extra fresh faces.
Flynn said on Johnson: "He hasn't had much of a pre-season.
"I was only going to give him 45 minutes, but I thought I'd push just to get the hour out of him and get him up to speed a bit more.
"He hasn't trained with the boys. He came last night. I met him at a hotel.
"So, it's pleasing to see Danny's performance. He adds that extra bit of experience and guile in the box."
Johnson is Walsall's 14th signing of the summer and Flynn insists he is not done there.
"I'm hoping to make some more signings this week, 100 per cent," said Flynn.
"Because of the injuries and how we are, you take the trialist out today and we're down to 16. That's not enough.
"The owners have been brilliant. It's an important part.
"We want to get this right and head into the season with just enough depth to get us through until a few of the injuries come back."
The Saddlers' victory at Solihull saw Johnson poke home towards the end of the first half.
Andy Williams made sure of the victory in the second period.
Asked if Walsall are ready for the start of the League Two season against Hartlepool United next weekend, Flynn added: "Fitness-wise, we are. There are a few things to work on next week.
"Solihull were excellent for the first 20 minutes. They're a good team with good players, have been together a while now and were unfortunate at the end of last season.
"It was a really good test as I don't think they'll lose many here all season.
"I want to give them credit as they gave us a real good test.
"We had to finish the game with 10 men as we couldn't risk Taylor (Allen) being injured. I was really proud of how they dug in.
"We had our 17-year-old attacking midfielder (Ronan Maher) at left-back. Flynn Clarke has probably never played on the left and had to double up as a second left-back.
"We dealt with the balls into the box. The attitude and how they threw their bodies on the line was pleasing to see.
"It's pleasing to see the two strikers score as well."