No, you cannot add VPOP3 licences together. If you have a 10 user licence, and you need more users, you need to upgrade to the next step (25 users in this case). You cannot buy a 5 user licence and add it to the existing 10 user licence.
This is because the price per user decreases rapidly as the licence sizes increase (eg £9 per user for 10 users, and £6 per user for 25 users). Some other licencing schemes have a fixed price per user licence so it makes more sense to add licences together, but when the price per user decreases for more users it is more cost effective to upgrade the licence size rather than to add licences.
For instance, if you have a 10 user licence, then need to upgrade to 20 users, if you were to add licences together, it would cost 2 x £90 = £180. On the other hand, upgrading the licence costs £90 (for the 10 user licence) + £70 (for the 10 → 25 upgrade) = £160 (plus you now have 25 users, instead of 20, so you won't need to upgrade again so soon).