Matt Derbyshire this week joined giant Serbian striker Nikola Zigic and goalkeeper Ben Foster as summer imports to St Andrew's after agreeing a season-long loan from Greek side Olympiacos.
McLeish said ahead of last weekend's 2-2 draw with Sunderland he was hopeful of bringing in new blood before Saturday's first home match of the season against Blackburn after missing out on Moussa Dembele, who has since joined Fulham.
City are one of a number of clubs to have been linked with Wigan wideman Charles N'Zogbia but, although he refused to confirm the names of his targets, the Scot assured fans there is plenty going on behind the scenes.
He said: "I'm trying to get the squad up to 25. We are working towards it for before the window closes. We still plan to bring a player or two in.
"We're making enquiries about certain situations and we've also had a big list of players, one or two that we've lost out on and one or two that we've decided to pass on and it'll keep going for a while.
"From last season's team photo I think there's 10 who have left the club so it's a case of filling the squad up and making sure it's not just numbers but the quality of people to augment the squad.
"I can't guarantee that everything will be in place but if it's not then that's a challenge for the players at the club to show that they can do what they did last year."
McLeish confirmed Derbyshire will make his debut against Rovers, the club he left for Olympiacos last January, after Birmingham received international clearance for the 24-year-old.
"He's in the squad," said the former Scotland boss. "We won't name the team until Friday but we must get Matt into the action. He's fit enough, he's looked bright in training and he'll be involved at some stage.
"He's got an eye for a goal and he's very sharp in and around the box. We believe we're getting a mature Matt Derbyshire and we're looking forward to working with him."
Blues netted twice in the final 12 minutes at the Stadium of Light to snatch what looked for most of the match an unlikely draw and McLeish made plain his disappointment at the performance.
Birmingham were the surprise package in the Barclays Premier League last season and their top-10 spot owed a lot to their tremendous home form, with Aston Villa and Bolton the only teams to leave with three points.
They go into Saturday's clash defending an 11-month unbeaten record on home soil, and McLeish is hoping they can pick up where they left off.
But he knows it will not be easy against a team who began the season in impressive fashion with a 1-0 victory over Everton last weekend.
He said: "We got away with a good point at a very difficult away venue. Sunderland played very well against us in the first half but I'd like to think there'll be a big improvement.
"Blackburn are a tough nut to crack. They had a fantastic result against Everton last week. Their confidence will be high. We've got to try to show the form we showed last season at St Andrew's."