Specific magic weapons have their own unique properties and generally cannot be improved further with item creation feats.
This is false; they can be. It is often difficult to determine exactly what the cost of doing so should be, but that’s a problem for the player and GM to work out together. The point is that the rules do let you do this.
See Upgrading Magic Items:
For specific magic armor and weapons, the price for the base item may be hard to determine, as some abilities may have been priced as plus-based properties and some as gp-based properties. Without knowing which is which, how to increase the price (using the plus-based table or flat gp addition) can’t be determined. If this happens and nobody can agree on a fair price, it’s best to not upgrade the item, or ask the GM for permission to pseudo-upgrade the item by swapping it for a different item with a price that can be calculated with the normal rules.
As for abilities like arcane pool, they do not mention any limit at +10. However, hiding in a parenthetical in the magic item rules is a single line stating
A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents, including those from character abilities and spells) higher than +10.
The “including those from character abilities and spells” here is a terrible rule. You should ignore it. Everyone should ignore it. Any time it comes up and is enforced, these abilities are badly nerfed—at the highest levels, when you already have a +10-equivalent weapon, these abilities become entirely worthless.
But even if you do use this rule, it still doesn’t act any differently for a specific magic weapon than it does for a regular one: you can still use it, you just have to come to an agreement with the DM about which parts of the weapon are due to enhancement-equivalent effects (and thus count towards the limit) and which come from flat costs (and thus do not count towards the limit). And since you do not have to actually price things, just be mindful of the limit, for the majority of the game you won’t even have to consider it, because your weapon will clearly be well under the +10 limit even if everything about it is considered enhancement-equivalent.