That is, if you mean to introduce the reference to the tool being used. 'kepeken' is a preposition and the characteristic of those words is that they always take their object immediately after them, without an intervening 'e', as verbs require.
That being said, there is a dialect of tp which harks back to a few years ago and which is still around in older textbooks and those derived from them, in which 'kepeken' is a verb or a verb and preposition. So, in this dialect, when 'kepeken' occupies the verb slot (right after 'li', etc.) it is a verb and requires 'e'. If it comes at the end, it is a preposition and doesn't.
But prepositions can go into the verb slot and, once there, still don't need 'e'. And the community came to notice that there was little difference between saying “He uses a tool for some unmentioned activity” and “He does some unmentioned activity using a tool” and so stuck with the simpler, no 'e', version throughout
It should also be noted that prepositions, like all non-verbs, can be used as transitive verbs in a causative sense. In that case, the Direct Object must, as usual, be introduced by 'e'. So, “I use a tool” is 'mi kepeken ilo' but “I make him use a tool” is “mi kepeken ilo e ona'
(Speaking of which, why, with all the drive to trim down the number of words in official tp, does it still have both 'kepeken' and 'ilo'?)