I really don't know if there were prisoners of war here. They were no captives here, everything they found will be killed once and for all including those who intend to come and help them. No one will escape this slaughter. The Hebrew ספה has been used as (Hiphil) meaning to say to catch up, gather hence join. This is a definition given by Brown-Driver-Briggs' Hebrew Definitions, Strong and Gesenius and The KjV translators are far more experts in the Hebrew language than you can imagine. The Latin versions as well favor the KJB more than what you are saying as a captive.
Vulgate
(i) 15 omnis qui inventus fuerit occidetur et omnis qui supervenerit cadet in gladio
Google translation of the Latin:
every one that is found shall be slain, and every one that cometh shall fall by the sword
Spanish Bible RV
ReinaValera
(i) 15 Cualquiera que fuere hallado, será alanceado; y cualquiera que á ellos se juntare, caerá á cuchillo.
Google:
Anyone found will be speared; and whoever
joins them will fall by the knife.
French Martin
Martin
(i) 15 Quiconque sera trouvé, sera transpercé; et quiconque s'y sera joint, tombera par l'épée.
Google Translator
Whoever is found will be pierced; and whoever joins in it will fall by the sword.
I bet this is enough to chew on.