Is a special charger required for Li-ION battery packs with a BMS?
Yes and no for your charger.
If you know the maximum intake of your in series connected pack you can use a regular psu.
But, it must cut off at the reached voltige...avoid a float charge.
You can also use a buck converter.
You can not use your 24v psu, simply the numbers won't add up, or you burn them or they won't charge.
Add up what you want to charge in series totally.
To give you a exact answer i would (must) like to know more about your batteries.