You are not obliged to use a OSCustomizationspec if you don't want/need to.
Just use something like this with the New-VM cmdlet
New-VM -Name MyVM -Template MyTemplate ....
And by adding the -VApp parameter you can add those new VMs to your virtual appliance.
Something like
New-vApp -Name MyvApp ...
New-VM -Name MyVM1 -Template MyTemplate1 -vApp MyvApp ....
New-VM -Name MyVM2 -Template MyTemplate2 -vApp MyvApp ....
New-VM -Name MyVM3 -Template MyTemplate3 -vApp MyvApp ...
....