It's not impossible to build a in-fill station, and construction can occur during operating hours.
North York Centre Station was built on the Yonge Line that way:
Quote:
Building a new station around a subway tunnel that is already in service posed unique challenges to construction. Yonge Street was excavated and decked over to minimize disruption to traffic. Workers had to dig around the subway tunnel to build the frame of the subway station’s two levels. The platform’s concrete floor could only be poured as far as the existing subway tunnel walls, for the workers had to wait until the concrete was properly set before those walls could be cut out. The cutting of the tunnel walls could only be done when the subway was shut down for the night. Once this was done, the remainder of the rider platforms were completed. The central wall between the two tracks was only opened up slightly and tiled over in the same basic white with blue accents as the rest of the station. Subway trains, passing through the station site, ran slow to protect construction workers, giving subway patrons a quick tour of the construction through their windows as they passed.
North York Centre Station, once opened, boasted a mezzanine level as well as a platform level. Escalators take passengers from the street level and the city centre development to the mezzanine level as well as between the mezzanine level and the platform. Construction was helped by the fact that a lot of the surrounding developments were under construction at the same time as the station, allowing workers to build access into surrounding buildings from the start, including North York City Centre.
North York Centre station opened on time on July 18, 1987 at a final cost of $25 million. Opening ceremonies were held at 10 a.m. with North York mayor Mel Lastman cutting the ribbon alongside Metro Chairman Dennis Flynn and Ontario Minister of Transportation Ed Fulton. The station officially opened to passengers at 2 p.m. that afternoon.
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https://transit.toronto.on.ca/subway...k-centre.shtml
For Dogwood Pearson, the main issue would be if an underpass below the existing tunnel is required.