You absolutely can tell Julie in HR to log into the company instance of The Lounge or whatever friendly frontend IT set up. Users should no more need to run their own bouncer than their own homeserver.
I'm sure Julie from HR also isn't present for every physical chat at the watercooler, and isn't any worse for wear.
In fact it'll be great for everyone if HR is not present at every watercooler conversation. At most companies we got work done despite company leadership and policy, not because :)
This assumes that there is actually a friendly frontend meeting the equivalent feature and UI/UX experiences of things like Slack and ignoring the cost of maintaining that service by IT.
Yes, I assume that https://thelounge.chat/ exists. As to cost/maintenance... there are no free lunches. You can pay with your money and your data, or your money and your time. I happen to find the ladder preferable and frankly cheaper than people think.