steppemum
Sorry to correct you a little!
The honorofic in Korean comes after the name not before eg 'JohnOppa' not 'OppaJohn' (oppa is big brother.)
In Korea, if someone is younger than you, you don't need to add any kind of honorific title. Just the name is fine, unless they are a total stranger, or if you have a fairly distant relationship, or at work, in which case you should add 'ssi'. Honorifics are for people older or higher in rank than you, there's no honorific like 'friend' or 'little sister', you just refer to such people by name.
There was really no reason why your colleagues couldn't have just asked you to use 'ssi' after their name, or to have used their job title, which is the common way to refer to people at work eg 'team leader' or 'director' or 'teacher' or whatever.
I don't think the Koreans using English names thing necessarily has anything to do with the use of honorifics. Sometimes it is used to get away from the rigid hierarchy of Korea when they're working with westerners - some Korean companies even use English names to avoid feeling like they can't make suggestions to those higher than them because the use of honorifics places barriers between you.
But the biggest part of it, in my opinion, is that they feel westerners can't learn their names, or if they do, they can't pronounce them properly. In my opinion, the first part is a real shame, because there is no reason not to learn Korean names - they are generally two syllables and have roughly the same phonemes as English. It's wuite easy to learn them, especially once you get used to common names like 'min' or 'woo'. As for feeling non-Koreans can't pronounce them - a lot have of Koreans have said this to me, but I don't get the problem, because it's not like the British can pronounce European names exactly correctly.
I think there is a degree of shame Koreans feel around their country too, as well as feeling that America is 'cooler' and that it is 'cool' to have an English name. I have noticed a trend with some younger Koreans keeping their names, so I hope this will contine.
Anyway, sorry to correct you...