普通にtimespanを足せば良いのでは
if ('20210531285930' -match '(\d{4})(\d\d)(\d\d)(\d\d)(\d\d)(\d\d)') {
$date = [datetime]::new($Matches[1],$Matches[2],$Matches[3]) + [timespan]::new($Matches[4],$Matches[5],$Matches[6])
}