You can always parallel more cells, as long as they are the same chemistry, and you add the same number of cells in parallel with each group.
You're simply adding more capacity and more current capability when you do this.
However, if you add series cells, you have to replace the BMS, or add a new BMS for just the new cells, and series the two BMSs. That can add complications to both charge and discharge, if anything unusual occurs (sometimes the FETs can be damaged during those unusual events). But it can also work fine. You have to use the same number and type and capacity (or higher) of cells, and same C-rate or current capability (or higher) for the group, or else the new part of the pack will limit the capability and capacity of the pack to whatever it's limitations are.
You're adding more watt-hours, so more capacity, but not changing the current capability of the pack. You are also increasing the voltage, which means the rest of the system has to be capable of that higher voltage (controller, charger, DC-DC if any, etc).
There is a teensy itsy bitsy chance that the BMS in the pack has unused channels...but that would cost the original builder/seller more and so is very unlikely to be the case.