@Maoira (I'm not an REI employee, but...) Probably not since the extra cog(s) add space, and the dropouts on the frame that the wheel goes into are a fixed distance, but even if you could you'd have to replace the shifters for sure and possibly the derailleur and a longer chain as well, it starts getting more reasonable to just wait until sizing up to a 24" bike for more gear range. You could try to find a different 6 speed cog set with more range, but there's only so far you can go with that before making the jumps between gears more awkward and clunky. If you do ever try that, it looks like it's actually a freewheel and not a cassette: the difference is minor on the outside, but major on the inside and how it attaches to the wheel hub, making the two system incompatible. And it still might need a different derailleur and chain to accommodate any larger cogs without over-stressing the chain.

Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.