Generally speaking you don't want to convert a pen that has both a threaded barrel and threaded cap into an eye dropper. Ideally you want threaded barrel, and snap/pull caps (like the pilot metro has... but not metal interior).
Why would you say that? When you say "threaded barrel", are you meaning that the section threads into the barrel itself? (One assumes as much)
Unless you have the rare pen where the cap threads on the front edge of the section, I see no reason why the two threaded areas would have any interaction, and therefore be of any caution. In fact, it seems that a snap on cap, which would be clamping onto the section, might be more of an issue - someone not used to the pen might try to unscrew the cap, and if it had a good grip on the section, might unscrew it and let loose the flood.