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M_V
Sorry clearly wasn't thinking straight when I replied first!
To re-answer: yes you can use a standard 1 1/8" top cap with them. Not that it should have any effect if you're running a quill but the allen bolt they (used to) give with those was barely long enough to meet the threads in the larger bolt it screws in to making setting preload on an ahead a real pain.
As an aside I thought those BBB extenstions come in two variants? A version with 22.2mm OD and then spacers that have a 25.4mm OD, or a version with 25.4mm OD and spacers with 26.8mm OD. So the 22.2mm OD lower end would fit in the 1" steerer but the 25.4mm spacers would then be equal to a standard 1" steerer OD ? So you'd either need to use a 1" ahead stem or a shim and 1 1/8" stem. All of which is easily done of course and superfluous to your original question.